Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

61 files changed, 7379 insertions, 0 deletions

diff --git a/intl/locale/AppDateTimeFormat.cpp b/intl/locale/AppDateTimeFormat.cpp
new file mode 100644
index 0000000000..d967d312a4
--- /dev/null
+++ b/intl/locale/AppDateTimeFormat.cpp
@@ -0,0 +1,263 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsCOMPtr.h"
+#include "mozilla/intl/AppDateTimeFormat.h"
+#include "mozilla/intl/DateTimePatternGenerator.h"
+#include "mozilla/intl/FormatBuffer.h"
+#include "mozilla/intl/LocaleService.h"
+#include "OSPreferences.h"
+#include "mozIOSPreferences.h"
+#ifdef DEBUG
+#  include "nsThreadManager.h"
+#endif
+
+namespace mozilla::intl {
+
+nsCString* AppDateTimeFormat::sLocale = nullptr;
+nsTHashMap<nsCStringHashKey, UniquePtr<DateTimeFormat>>*
+    AppDateTimeFormat::sFormatCache;
+
+static const int32_t DATETIME_FORMAT_INITIAL_LEN = 127;
+
+/*static*/
+nsresult AppDateTimeFormat::Initialize() {
+  MOZ_ASSERT(NS_IsMainThread());
+  if (sLocale) {
+    return NS_OK;
+  }
+
+  sLocale = new nsCString();
+  AutoTArray<nsCString, 10> regionalPrefsLocales;
+  LocaleService::GetInstance()->GetRegionalPrefsLocales(regionalPrefsLocales);
+  sLocale->Assign(regionalPrefsLocales[0]);
+
+  return NS_OK;
+}
+
+// performs a locale sensitive date formatting operation on the PRTime parameter
+/*static*/
+nsresult AppDateTimeFormat::Format(const DateTimeFormat::StyleBag& aStyle,
+                                   const PRTime aPrTime,
+                                   nsAString& aStringOut) {
+  return AppDateTimeFormat::Format(
+      aStyle, (static_cast<double>(aPrTime) / PR_USEC_PER_MSEC), nullptr,
+      aStringOut);
+}
+
+// performs a locale sensitive date formatting operation on the PRExplodedTime
+// parameter
+/*static*/
+nsresult AppDateTimeFormat::Format(const DateTimeFormat::StyleBag& aStyle,
+                                   const PRExplodedTime* aExplodedTime,
+                                   nsAString& aStringOut) {
+  return AppDateTimeFormat::Format(
+      aStyle, (PR_ImplodeTime(aExplodedTime) / PR_USEC_PER_MSEC),
+      &(aExplodedTime->tm_params), aStringOut);
+}
+
+// performs a locale sensitive date formatting operation on the PRExplodedTime
+// parameter, using the specified options.
+/*static*/
+nsresult AppDateTimeFormat::Format(const DateTimeFormat::ComponentsBag& aBag,
+                                   const PRExplodedTime* aExplodedTime,
+                                   nsAString& aStringOut) {
+  // set up locale data
+  nsresult rv = Initialize();
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  aStringOut.Truncate();
+
+  nsAutoCString str;
+  nsAutoString timeZoneID;
+  BuildTimeZoneString(aExplodedTime->tm_params, timeZoneID);
+
+  auto genResult = DateTimePatternGenerator::TryCreate(sLocale->get());
+  NS_ENSURE_TRUE(genResult.isOk(), NS_ERROR_FAILURE);
+  auto dateTimePatternGenerator = genResult.unwrap();
+
+  auto result = DateTimeFormat::TryCreateFromComponents(
+      *sLocale, aBag, dateTimePatternGenerator.get(), Some(timeZoneID));
+  NS_ENSURE_TRUE(result.isOk(), NS_ERROR_FAILURE);
+  auto dateTimeFormat = result.unwrap();
+
+  double unixEpoch =
+      static_cast<float>((PR_ImplodeTime(aExplodedTime) / PR_USEC_PER_MSEC));
+
+  aStringOut.SetLength(DATETIME_FORMAT_INITIAL_LEN);
+  nsTStringToBufferAdapter buffer(aStringOut);
+  NS_ENSURE_TRUE(dateTimeFormat->TryFormat(unixEpoch, buffer).isOk(),
+                 NS_ERROR_FAILURE);
+
+  return rv;
+}
+
+/**
+ * An internal utility function to serialize a Maybe<DateTimeFormat::Style> to
+ * an int, to be used as a caching key.
+ */
+static int StyleToInt(const Maybe<DateTimeFormat::Style>& aStyle) {
+  if (aStyle.isSome()) {
+    switch (*aStyle) {
+      case DateTimeFormat::Style::Full:
+        return 1;
+      case DateTimeFormat::Style::Long:
+        return 2;
+      case DateTimeFormat::Style::Medium:
+        return 3;
+      case DateTimeFormat::Style::Short:
+        return 4;
+    }
+  }
+  return 0;
+}
+
+/*static*/
+nsresult AppDateTimeFormat::Format(const DateTimeFormat::StyleBag& aStyle,
+                                   const double aUnixEpoch,
+                                   const PRTimeParameters* aTimeParameters,
+                                   nsAString& aStringOut) {
+  nsresult rv = NS_OK;
+
+  // return, nothing to format
+  if (aStyle.date.isNothing() && aStyle.time.isNothing()) {
+    aStringOut.Truncate();
+    return NS_OK;
+  }
+
+  // set up locale data
+  rv = Initialize();
+
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  nsAutoCString key;
+  key.AppendInt(StyleToInt(aStyle.date));
+  key.Append(':');
+  key.AppendInt(StyleToInt(aStyle.time));
+  if (aTimeParameters) {
+    key.Append(':');
+    key.AppendInt(aTimeParameters->tp_gmt_offset);
+    key.Append(':');
+    key.AppendInt(aTimeParameters->tp_dst_offset);
+  }
+
+  if (sFormatCache && sFormatCache->Count() == kMaxCachedFormats) {
+    // Don't allow a pathological page to extend the cache unreasonably.
+    NS_WARNING("flushing DateTimeFormat cache");
+    DeleteCache();
+  }
+  if (!sFormatCache) {
+    sFormatCache = new nsTHashMap<nsCStringHashKey, UniquePtr<DateTimeFormat>>(
+        kMaxCachedFormats);
+  }
+
+  UniquePtr<DateTimeFormat>& dateTimeFormat = sFormatCache->LookupOrInsert(key);
+
+  if (!dateTimeFormat) {
+    // We didn't have a cached formatter for this key, so create one.
+    int32_t dateFormatStyle = mozIOSPreferences::dateTimeFormatStyleNone;
+    if (aStyle.date.isSome()) {
+      switch (*aStyle.date) {
+        case DateTimeFormat::Style::Full:
+        case DateTimeFormat::Style::Long:
+          dateFormatStyle = mozIOSPreferences::dateTimeFormatStyleLong;
+          break;
+        case DateTimeFormat::Style::Medium:
+        case DateTimeFormat::Style::Short:
+          dateFormatStyle = mozIOSPreferences::dateTimeFormatStyleShort;
+          break;
+      }
+    }
+
+    int32_t timeFormatStyle = mozIOSPreferences::dateTimeFormatStyleNone;
+    if (aStyle.time.isSome()) {
+      switch (*aStyle.time) {
+        case DateTimeFormat::Style::Full:
+        case DateTimeFormat::Style::Long:
+          timeFormatStyle = mozIOSPreferences::dateTimeFormatStyleLong;
+          break;
+        case DateTimeFormat::Style::Medium:
+        case DateTimeFormat::Style::Short:
+          timeFormatStyle = mozIOSPreferences::dateTimeFormatStyleShort;
+          break;
+      }
+    }
+
+    nsAutoCString str;
+    rv = OSPreferences::GetInstance()->GetDateTimePattern(
+        dateFormatStyle, timeFormatStyle, nsDependentCString(sLocale->get()),
+        str);
+    NS_ENSURE_SUCCESS(rv, rv);
+    nsAutoString pattern = NS_ConvertUTF8toUTF16(str);
+
+    Maybe<Span<const char16_t>> timeZoneOverride = Nothing();
+    nsAutoString timeZoneID;
+    if (aTimeParameters) {
+      BuildTimeZoneString(*aTimeParameters, timeZoneID);
+      timeZoneOverride =
+          Some(Span<const char16_t>(timeZoneID.Data(), timeZoneID.Length()));
+    }
+
+    auto result = DateTimeFormat::TryCreateFromPattern(*sLocale, pattern,
+                                                       timeZoneOverride);
+    NS_ENSURE_TRUE(result.isOk(), NS_ERROR_FAILURE);
+    dateTimeFormat = result.unwrap();
+  }
+
+  MOZ_ASSERT(dateTimeFormat);
+
+  aStringOut.SetLength(DATETIME_FORMAT_INITIAL_LEN);
+  nsTStringToBufferAdapter buffer(aStringOut);
+  NS_ENSURE_TRUE(dateTimeFormat->TryFormat(aUnixEpoch, buffer).isOk(),
+                 NS_ERROR_FAILURE);
+
+  return rv;
+}
+
+/*static*/
+void AppDateTimeFormat::BuildTimeZoneString(
+    const PRTimeParameters& aTimeParameters, nsAString& aStringOut) {
+  aStringOut.Truncate();
+  aStringOut.Append(u"GMT");
+  int32_t totalOffsetMinutes =
+      (aTimeParameters.tp_gmt_offset + aTimeParameters.tp_dst_offset) / 60;
+  if (totalOffsetMinutes != 0) {
+    char sign = totalOffsetMinutes < 0 ? '-' : '+';
+    int32_t hours = abs(totalOffsetMinutes) / 60;
+    int32_t minutes = abs(totalOffsetMinutes) % 60;
+    aStringOut.AppendPrintf("%c%02d:%02d", sign, hours, minutes);
+  }
+}
+
+/*static*/
+void AppDateTimeFormat::DeleteCache() {
+  MOZ_ASSERT(NS_IsMainThread());
+  if (sFormatCache) {
+    delete sFormatCache;
+    sFormatCache = nullptr;
+  }
+}
+
+/*static*/
+void AppDateTimeFormat::Shutdown() {
+  MOZ_ASSERT(NS_IsMainThread());
+  DeleteCache();
+  delete sLocale;
+}
+
+/*static*/
+void AppDateTimeFormat::ClearLocaleCache() {
+  MOZ_ASSERT(NS_IsMainThread());
+  DeleteCache();
+  delete sLocale;
+  sLocale = nullptr;
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/locale/AppDateTimeFormat.h b/intl/locale/AppDateTimeFormat.h
new file mode 100644
index 0000000000..cdc8a499bb
--- /dev/null
+++ b/intl/locale/AppDateTimeFormat.h
@@ -0,0 +1,89 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_intl_AppDateTimeFormat_h
+#define mozilla_intl_AppDateTimeFormat_h
+
+#include <time.h>
+#include "gtest/MozGtestFriend.h"
+#include "nsTHashMap.h"
+#include "nsString.h"
+#include "prtime.h"
+#include "mozilla/intl/DateTimeFormat.h"
+
+namespace mozilla::intl {
+
+/**
+ * Get a DateTimeFormat for use in Gecko. This specialized DateTimeFormat
+ * respects the user's OS and app preferences, and provides caching of the
+ * underlying mozilla::intl resources.
+ *
+ * This class is not thread-safe as it lazily initializes a cache without
+ * any type of multi-threaded protections.
+ */
+class AppDateTimeFormat {
+ public:
+  /**
+   * Format a DateTime using the applied app and OS-level preferences, with a
+   * style bag and the PRTime.
+   */
+  static nsresult Format(const DateTimeFormat::StyleBag& aStyle,
+                         const PRTime aPrTime, nsAString& aStringOut);
+
+  /**
+   * Format a DateTime using the applied app and OS-level preferences, with a
+   * style bag and the PRExplodedTime.
+   */
+  static nsresult Format(const DateTimeFormat::StyleBag& aStyle,
+                         const PRExplodedTime* aExplodedTime,
+                         nsAString& aStringOut);
+
+  /**
+   * Format a DateTime using the applied app and OS-level preferences, with a
+   * components bag and the PRExplodedTime.
+   */
+  static nsresult Format(const DateTimeFormat::ComponentsBag& aComponents,
+                         const PRExplodedTime* aExplodedTime,
+                         nsAString& aStringOut);
+
+  /**
+   * If the app locale changes, the cached locale needs to be reset.
+   */
+  static void ClearLocaleCache();
+
+  static void Shutdown();
+
+ private:
+  AppDateTimeFormat() = delete;
+
+  static nsresult Initialize();
+  static void DeleteCache();
+  static const size_t kMaxCachedFormats = 15;
+
+  FRIEND_TEST(AppDateTimeFormat, FormatPRExplodedTime);
+  FRIEND_TEST(AppDateTimeFormat, DateFormatSelectors);
+  FRIEND_TEST(AppDateTimeFormat, FormatPRExplodedTimeForeign);
+  FRIEND_TEST(AppDateTimeFormat, DateFormatSelectorsForeign);
+
+  /**
+   * Format a DateTime using the applied app and OS-level preferences, with a
+   * components bag and the PRExplodedTime.
+   */
+  static nsresult Format(const DateTimeFormat::StyleBag& aStyle,
+                         const double aUnixEpoch,
+                         const PRTimeParameters* aTimeParameters,
+                         nsAString& aStringOut);
+
+  static void BuildTimeZoneString(const PRTimeParameters& aTimeParameters,
+                                  nsAString& aStringOut);
+
+  static nsCString* sLocale;
+  static nsTHashMap<nsCStringHashKey, UniquePtr<DateTimeFormat>>* sFormatCache;
+};
+
+}  // namespace mozilla::intl
+
+#endif /* mozilla_intl_AppDateTimeFormat_h */
diff --git a/intl/locale/LangPackMatcher.sys.mjs b/intl/locale/LangPackMatcher.sys.mjs
new file mode 100644
index 0000000000..977398b082
--- /dev/null
+++ b/intl/locale/LangPackMatcher.sys.mjs
@@ -0,0 +1,395 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AddonManager: "resource://gre/modules/AddonManager.sys.mjs",
+  AddonRepository: "resource://gre/modules/addons/AddonRepository.sys.mjs",
+});
+
+if (Services.appinfo.processType !== Services.appinfo.PROCESS_TYPE_DEFAULT) {
+  // This check ensures that the `mockable` API calls can be consisently mocked in tests.
+  // If this requirement needs to be eased, please ensure the test logic remains valid.
+  throw new Error("This code is assumed to run in the parent process.");
+}
+
+/**
+ * Attempts to find an appropriate langpack for a given language. The async function
+ * is infallible, but may not return a langpack.
+ *
+ * @returns {{
+ *   langPack: LangPack | null,
+ *   langPackDisplayName: string | null
+ * }}
+ */
+async function negotiateLangPackForLanguageMismatch() {
+  const localeInfo = getAppAndSystemLocaleInfo();
+  const nullResult = {
+    langPack: null,
+    langPackDisplayName: null,
+  };
+  if (!localeInfo.systemLocale) {
+    // The system locale info was not valid.
+    return nullResult;
+  }
+
+  /**
+   * Fetch the available langpacks from AMO.
+   *
+   * @type {Array<LangPack>}
+   */
+  const availableLangpacks = await mockable.getAvailableLangpacks();
+  if (!availableLangpacks) {
+    return nullResult;
+  }
+
+  /**
+   * Figure out a langpack to recommend.
+   * @type {LangPack | null}
+   */
+  const langPack =
+    // First look for a langpack that matches the baseName, which may include a script.
+    // e.g. system "fr-FR" matches langpack "fr-FR"
+    //      system "en-GB" matches langpack "en-GB".
+    //      system "zh-Hant-CN" matches langpack "zh-Hant-CN".
+    availableLangpacks.find(
+      ({ target_locale }) => target_locale === localeInfo.systemLocale.baseName
+    ) ||
+    // Next try matching language and region while excluding script
+    // e.g. system "zh-Hant-TW" matches langpack "zh-TW" but not "zh-CN".
+    availableLangpacks.find(
+      ({ target_locale }) =>
+        target_locale ===
+        `${localeInfo.systemLocale.language}-${localeInfo.systemLocale.region}`
+    ) ||
+    // Next look for langpacks that just match the language.
+    // e.g. system "fr-FR" matches langpack "fr".
+    //      system "en-AU" matches langpack "en".
+    availableLangpacks.find(
+      ({ target_locale }) => target_locale === localeInfo.systemLocale.language
+    ) ||
+    // Next look for a langpack that matches the language, but not the region.
+    // e.g. "es-CL" (Chilean Spanish) as a system language matching
+    //      "es-ES" (European Spanish)
+    availableLangpacks.find(({ target_locale }) =>
+      target_locale.startsWith(`${localeInfo.systemLocale.language}-`)
+    ) ||
+    null;
+
+  if (!langPack) {
+    return nullResult;
+  }
+
+  return {
+    langPack,
+    langPackDisplayName: Services.intl.getLocaleDisplayNames(
+      undefined,
+      [langPack.target_locale],
+      { preferNative: true }
+    )[0],
+  };
+}
+
+// If a langpack is being installed, allow blocking on that.
+let installingLangpack = new Map();
+
+/**
+ * @typedef {LangPack}
+ * @type {object}
+ * @property {string} target_locale
+ * @property {string} url
+ * @property {string} hash
+ */
+
+/**
+ * Ensure that a given lanpack is installed.
+ *
+ * @param {LangPack} langPack
+ * @returns {Promise<boolean>} Success or failure.
+ */
+function ensureLangPackInstalled(langPack) {
+  if (!langPack) {
+    throw new Error("Expected a LangPack to install.");
+  }
+  // Make sure any outstanding calls get resolved before attempting another call.
+  // This guards against any quick page refreshes attempting to install the langpack
+  // twice.
+  const inProgress = installingLangpack.get(langPack.hash);
+  if (inProgress) {
+    return inProgress;
+  }
+  const promise = _ensureLangPackInstalledImpl(langPack);
+  installingLangpack.set(langPack.hash, promise);
+  promise.finally(() => {
+    installingLangpack.delete(langPack.hash);
+  });
+  return promise;
+}
+
+/**
+ * @param {LangPack} langPack
+ * @returns {boolean} Success or failure.
+ */
+async function _ensureLangPackInstalledImpl(langPack) {
+  const availablelocales = await getAvailableLocales();
+  if (availablelocales.includes(langPack.target_locale)) {
+    // The langpack is already installed.
+    return true;
+  }
+
+  return mockable.installLangPack(langPack);
+}
+
+/**
+ * These are all functions with side effects or configuration options that should be
+ * mockable for tests.
+ */
+const mockable = {
+  /**
+   * @returns {LangPack[] | null}
+   */
+  async getAvailableLangpacks() {
+    try {
+      return lazy.AddonRepository.getAvailableLangpacks();
+    } catch (error) {
+      console.error(
+        `Failed to get the list of available language packs: ${error?.message}`
+      );
+      return null;
+    }
+  },
+
+  /**
+   * Use the AddonManager to install an addon from the URL.
+   * @param {LangPack} langPack
+   */
+  async installLangPack(langPack) {
+    let install;
+    try {
+      install = await lazy.AddonManager.getInstallForURL(langPack.url, {
+        hash: langPack.hash,
+        telemetryInfo: {
+          source: "about:welcome",
+        },
+      });
+    } catch (error) {
+      console.error(error);
+      return false;
+    }
+
+    try {
+      await install.install();
+    } catch (error) {
+      console.error(error);
+      return false;
+    }
+    return true;
+  },
+
+  /**
+   * Returns the available locales, including the fallback locale, which may not include
+   * all of the resources, in cases where the defaultLocale is not "en-US".
+   *
+   * @returns {string[]}
+   */
+  getAvailableLocalesIncludingFallback() {
+    return Services.locale.availableLocales;
+  },
+
+  /**
+   * @returns {string}
+   */
+  getDefaultLocale() {
+    return Services.locale.defaultLocale;
+  },
+
+  /**
+   * @returns {string}
+   */
+  getLastFallbackLocale() {
+    return Services.locale.lastFallbackLocale;
+  },
+
+  /**
+   * @returns {string}
+   */
+  getAppLocaleAsBCP47() {
+    return Services.locale.appLocaleAsBCP47;
+  },
+
+  /**
+   * @returns {string}
+   */
+  getSystemLocale() {
+    // Allow the system locale to be overridden for manual testing.
+    const systemLocaleOverride = Services.prefs.getCharPref(
+      "intl.multilingual.aboutWelcome.systemLocaleOverride",
+      null
+    );
+    if (systemLocaleOverride) {
+      try {
+        // If the locale can't be parsed, ignore the pref.
+        new Services.intl.Locale(systemLocaleOverride);
+        return systemLocaleOverride;
+      } catch (_error) {}
+    }
+
+    const osPrefs = Cc["@mozilla.org/intl/ospreferences;1"].getService(
+      Ci.mozIOSPreferences
+    );
+    return osPrefs.systemLocale;
+  },
+
+  /**
+   * @param {string[]} locales The BCP 47 locale identifiers.
+   */
+  setRequestedAppLocales(locales) {
+    Services.locale.requestedLocales = locales;
+  },
+};
+
+/**
+ * This function is really only setting `Services.locale.requestedLocales`, but it's
+ * using the `mockable` object to allow this behavior to be mocked in tests.
+ *
+ * @param {string[]} locales The BCP 47 locale identifiers.
+ */
+function setRequestedAppLocales(locales) {
+  mockable.setRequestedAppLocales(locales);
+}
+
+/**
+ * A serializable Intl.Locale.
+ *
+ * @typedef StructuredLocale
+ * @type {object}
+ * @property {string} baseName
+ * @property {string} language
+ * @property {string} region
+ */
+
+/**
+ * In telemetry data, some of the system locales show up as blank. Guard against this
+ * and any other malformed locale information provided by the system by wrapping the call
+ * into a catch/try.
+ *
+ * @param {string} locale
+ * @returns {StructuredLocale | null}
+ */
+function getStructuredLocaleOrNull(localeString) {
+  try {
+    const locale = new Services.intl.Locale(localeString);
+    return {
+      baseName: locale.baseName,
+      language: locale.language,
+      region: locale.region,
+    };
+  } catch (_err) {
+    return null;
+  }
+}
+
+/**
+ * Determine the system and app locales, and how much the locales match.
+ *
+ * @returns {{
+ *  systemLocale: StructuredLocale,
+ *  appLocale: StructuredLocale,
+ *  matchType: "unknown" | "language-mismatch" | "region-mismatch" | "match",
+ * }}
+ */
+function getAppAndSystemLocaleInfo() {
+  // Convert locale strings into structured locale objects.
+  const systemLocaleRaw = mockable.getSystemLocale();
+  const appLocaleRaw = mockable.getAppLocaleAsBCP47();
+
+  const systemLocale = getStructuredLocaleOrNull(systemLocaleRaw);
+  const appLocale = getStructuredLocaleOrNull(appLocaleRaw);
+
+  let matchType = "unknown";
+  if (systemLocale && appLocale) {
+    if (systemLocale.language !== appLocale.language) {
+      matchType = "language-mismatch";
+    } else if (systemLocale.region !== appLocale.region) {
+      matchType = "region-mismatch";
+    } else {
+      matchType = "match";
+    }
+  }
+
+  // Live reloading with bidi switching may not be supported.
+  let canLiveReload = null;
+  if (systemLocale && appLocale) {
+    const systemDirection = Services.intl.getScriptDirection(
+      systemLocale.language
+    );
+    const appDirection = Services.intl.getScriptDirection(appLocale.language);
+    const supportsBidiSwitching = Services.prefs.getBoolPref(
+      "intl.multilingual.liveReloadBidirectional",
+      false
+    );
+    canLiveReload = systemDirection === appDirection || supportsBidiSwitching;
+  }
+  return {
+    // Return the Intl.Locale in a serializable form.
+    systemLocaleRaw,
+    systemLocale,
+    appLocaleRaw,
+    appLocale,
+    matchType,
+    canLiveReload,
+
+    // These can be used as Fluent message args.
+    displayNames: {
+      systemLanguage: systemLocale
+        ? Services.intl.getLocaleDisplayNames(
+            undefined,
+            [systemLocale.baseName],
+            { preferNative: true }
+          )[0]
+        : null,
+      appLanguage: appLocale
+        ? Services.intl.getLocaleDisplayNames(undefined, [appLocale.baseName], {
+            preferNative: true,
+          })[0]
+        : null,
+    },
+  };
+}
+
+/**
+ * Filter the lastFallbackLocale from availableLocales if it doesn't have all
+ * of the needed strings.
+ *
+ * When the lastFallbackLocale isn't the defaultLocale, then by default only
+ * fluent strings are included. To fully use that locale you need the langpack
+ * to be installed, so if it isn't installed remove it from availableLocales.
+ */
+async function getAvailableLocales() {
+  const availableLocales = mockable.getAvailableLocalesIncludingFallback();
+  const defaultLocale = mockable.getDefaultLocale();
+  const lastFallbackLocale = mockable.getLastFallbackLocale();
+  // If defaultLocale isn't lastFallbackLocale, then we still need the langpack
+  // for lastFallbackLocale for it to be useful.
+  if (defaultLocale != lastFallbackLocale) {
+    let lastFallbackId = `langpack-${lastFallbackLocale}@firefox.mozilla.org`;
+    let lastFallbackInstalled = await lazy.AddonManager.getAddonByID(
+      lastFallbackId
+    );
+    if (!lastFallbackInstalled) {
+      return availableLocales.filter(locale => locale != lastFallbackLocale);
+    }
+  }
+  return availableLocales;
+}
+
+export var LangPackMatcher = {
+  negotiateLangPackForLanguageMismatch,
+  ensureLangPackInstalled,
+  getAppAndSystemLocaleInfo,
+  setRequestedAppLocales,
+  getAvailableLocales,
+  mockable,
+};
diff --git a/intl/locale/LocaleService.cpp b/intl/locale/LocaleService.cpp
new file mode 100644
index 0000000000..1527f27910
--- /dev/null
+++ b/intl/locale/LocaleService.cpp
@@ -0,0 +1,693 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "LocaleService.h"
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Omnijar.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/Services.h"
+#include "mozilla/StaticPrefs_privacy.h"
+#include "mozilla/intl/AppDateTimeFormat.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/intl/OSPreferences.h"
+#include "nsDirectoryService.h"
+#include "nsDirectoryServiceDefs.h"
+#include "nsIObserverService.h"
+#include "nsStringEnumerator.h"
+#include "nsXULAppAPI.h"
+#include "nsZipArchive.h"
+#ifdef XP_WIN
+#  include "WinUtils.h"
+#endif
+#ifdef MOZ_WIDGET_GTK
+#  include "mozilla/WidgetUtilsGtk.h"
+#endif
+
+#define INTL_SYSTEM_LOCALES_CHANGED "intl:system-locales-changed"
+
+#define PSEUDO_LOCALE_PREF "intl.l10n.pseudo"
+#define REQUESTED_LOCALES_PREF "intl.locale.requested"
+#define WEB_EXPOSED_LOCALES_PREF "intl.locale.privacy.web_exposed"
+
+static const char* kObservedPrefs[] = {REQUESTED_LOCALES_PREF,
+                                       WEB_EXPOSED_LOCALES_PREF,
+                                       PSEUDO_LOCALE_PREF, nullptr};
+
+using namespace mozilla::intl::ffi;
+using namespace mozilla::intl;
+using namespace mozilla;
+
+NS_IMPL_ISUPPORTS(LocaleService, mozILocaleService, nsIObserver,
+                  nsISupportsWeakReference)
+
+mozilla::StaticRefPtr<LocaleService> LocaleService::sInstance;
+
+/**
+ * This function splits an input string by `,` delimiter, sanitizes the result
+ * language tags and returns them to the caller.
+ */
+static void SplitLocaleListStringIntoArray(nsACString& str,
+                                           nsTArray<nsCString>& aRetVal) {
+  if (str.Length() > 0) {
+    for (const nsACString& part : str.Split(',')) {
+      nsAutoCString locale(part);
+      if (LocaleService::CanonicalizeLanguageId(locale)) {
+        if (!aRetVal.Contains(locale)) {
+          aRetVal.AppendElement(locale);
+        }
+      }
+    }
+  }
+}
+
+static void ReadRequestedLocales(nsTArray<nsCString>& aRetVal) {
+  nsAutoCString str;
+  nsresult rv = Preferences::GetCString(REQUESTED_LOCALES_PREF, str);
+  // isRepack means this is a version of Firefox specifically
+  // built for one language.
+  const bool isRepack =
+#ifdef XP_WIN
+      !mozilla::widget::WinUtils::HasPackageIdentity();
+#elif defined(MOZ_WIDGET_GTK)
+      !widget::IsRunningUnderSnap();
+#else
+      true;
+#endif
+
+  // We handle four scenarios here:
+  //
+  // 1) The pref is not set - use default locale
+  // 2) The pref is not set and we're a packaged app - use OS locales
+  // 3) The pref is set to "" - use OS locales
+  // 4) The pref is set to a value - parse the locale list and use it
+  if (NS_SUCCEEDED(rv)) {
+    if (str.Length() == 0) {
+      // Case 3
+      OSPreferences::GetInstance()->GetSystemLocales(aRetVal);
+    } else {
+      // Case 4
+      SplitLocaleListStringIntoArray(str, aRetVal);
+    }
+  }
+
+  // This will happen when either the pref is not set,
+  // or parsing of the pref didn't produce any usable
+  // result.
+  if (aRetVal.IsEmpty()) {
+    if (isRepack) {
+      // Case 1
+      nsAutoCString defaultLocale;
+      LocaleService::GetInstance()->GetDefaultLocale(defaultLocale);
+      aRetVal.AppendElement(defaultLocale);
+    } else {
+      // Case 2
+      OSPreferences::GetInstance()->GetSystemLocales(aRetVal);
+    }
+  }
+}
+
+static void ReadWebExposedLocales(nsTArray<nsCString>& aRetVal) {
+  nsAutoCString str;
+  nsresult rv = Preferences::GetCString(WEB_EXPOSED_LOCALES_PREF, str);
+  if (NS_WARN_IF(NS_FAILED(rv)) || str.Length() == 0) {
+    return;
+  }
+
+  SplitLocaleListStringIntoArray(str, aRetVal);
+}
+
+LocaleService::LocaleService(bool aIsServer) : mIsServer(aIsServer) {}
+
+/**
+ * This function performs the actual language negotiation for the API.
+ *
+ * Currently it collects the locale ID used by nsChromeRegistry and
+ * adds hardcoded default locale as a fallback.
+ */
+void LocaleService::NegotiateAppLocales(nsTArray<nsCString>& aRetVal) {
+  if (mIsServer) {
+    nsAutoCString defaultLocale;
+    AutoTArray<nsCString, 100> availableLocales;
+    AutoTArray<nsCString, 10> requestedLocales;
+    GetDefaultLocale(defaultLocale);
+    GetAvailableLocales(availableLocales);
+    GetRequestedLocales(requestedLocales);
+
+    NegotiateLanguages(requestedLocales, availableLocales, defaultLocale,
+                       kLangNegStrategyFiltering, aRetVal);
+  }
+
+  nsAutoCString lastFallbackLocale;
+  GetLastFallbackLocale(lastFallbackLocale);
+
+  if (!aRetVal.Contains(lastFallbackLocale)) {
+    // This part is used in one of the two scenarios:
+    //
+    // a) We're in a client mode, and no locale has been set yet,
+    //    so we need to return last fallback locale temporarily.
+    // b) We're in a server mode, and the last fallback locale was excluded
+    //    when negotiating against the requested locales.
+    //    Since we currently package it as a last fallback at build
+    //    time, we should also add it at the end of the list at
+    //    runtime.
+    aRetVal.AppendElement(lastFallbackLocale);
+  }
+}
+
+LocaleService* LocaleService::GetInstance() {
+  if (!sInstance) {
+    sInstance = new LocaleService(XRE_IsParentProcess());
+
+    if (sInstance->IsServer()) {
+      // We're going to observe for requested languages changes which come
+      // from prefs.
+      DebugOnly<nsresult> rv =
+          Preferences::AddWeakObservers(sInstance, kObservedPrefs);
+      MOZ_ASSERT(NS_SUCCEEDED(rv), "Adding observers failed.");
+
+      nsCOMPtr<nsIObserverService> obs =
+          mozilla::services::GetObserverService();
+      if (obs) {
+        obs->AddObserver(sInstance, INTL_SYSTEM_LOCALES_CHANGED, true);
+        obs->AddObserver(sInstance, NS_XPCOM_SHUTDOWN_OBSERVER_ID, true);
+      }
+    }
+    // DOM might use ICUUtils and LocaleService during UnbindFromTree by
+    // final cycle collection.
+    ClearOnShutdown(&sInstance, ShutdownPhase::CCPostLastCycleCollection);
+  }
+  return sInstance;
+}
+
+static void NotifyAppLocaleChanged() {
+  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+  if (obs) {
+    obs->NotifyObservers(nullptr, "intl:app-locales-changed", nullptr);
+  }
+  // The locale in AppDateTimeFormat is cached statically.
+  AppDateTimeFormat::ClearLocaleCache();
+}
+
+void LocaleService::RemoveObservers() {
+  if (mIsServer) {
+    Preferences::RemoveObservers(this, kObservedPrefs);
+
+    nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+    if (obs) {
+      obs->RemoveObserver(this, INTL_SYSTEM_LOCALES_CHANGED);
+      obs->RemoveObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID);
+    }
+  }
+}
+
+void LocaleService::AssignAppLocales(const nsTArray<nsCString>& aAppLocales) {
+  MOZ_ASSERT(!mIsServer,
+             "This should only be called for LocaleService in client mode.");
+
+  mAppLocales = aAppLocales.Clone();
+  NotifyAppLocaleChanged();
+}
+
+void LocaleService::AssignRequestedLocales(
+    const nsTArray<nsCString>& aRequestedLocales) {
+  MOZ_ASSERT(!mIsServer,
+             "This should only be called for LocaleService in client mode.");
+
+  mRequestedLocales = aRequestedLocales.Clone();
+  nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+  if (obs) {
+    obs->NotifyObservers(nullptr, "intl:requested-locales-changed", nullptr);
+  }
+}
+
+void LocaleService::RequestedLocalesChanged() {
+  MOZ_ASSERT(mIsServer, "This should only be called in the server mode.");
+
+  nsTArray<nsCString> newLocales;
+  ReadRequestedLocales(newLocales);
+
+  if (mRequestedLocales != newLocales) {
+    mRequestedLocales = std::move(newLocales);
+    nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+    if (obs) {
+      obs->NotifyObservers(nullptr, "intl:requested-locales-changed", nullptr);
+    }
+    LocalesChanged();
+  }
+}
+
+void LocaleService::WebExposedLocalesChanged() {
+  MOZ_ASSERT(mIsServer, "This should only be called in the server mode.");
+
+  nsTArray<nsCString> newLocales;
+  ReadWebExposedLocales(newLocales);
+  if (mWebExposedLocales != newLocales) {
+    mWebExposedLocales = std::move(newLocales);
+  }
+}
+
+void LocaleService::LocalesChanged() {
+  MOZ_ASSERT(mIsServer, "This should only be called in the server mode.");
+
+  // if mAppLocales has not been initialized yet, just return
+  if (mAppLocales.IsEmpty()) {
+    return;
+  }
+
+  nsTArray<nsCString> newLocales;
+  NegotiateAppLocales(newLocales);
+
+  if (mAppLocales != newLocales) {
+    mAppLocales = std::move(newLocales);
+    NotifyAppLocaleChanged();
+  }
+}
+
+bool LocaleService::IsLocaleRTL(const nsACString& aLocale) {
+  return unic_langid_is_rtl(&aLocale);
+}
+
+bool LocaleService::IsAppLocaleRTL() {
+  // Next, check if there is a pseudo locale `bidi` set.
+  nsAutoCString pseudoLocale;
+  if (NS_SUCCEEDED(Preferences::GetCString("intl.l10n.pseudo", pseudoLocale))) {
+    if (pseudoLocale.EqualsLiteral("bidi")) {
+      return true;
+    }
+    if (pseudoLocale.EqualsLiteral("accented")) {
+      return false;
+    }
+  }
+
+  nsAutoCString locale;
+  GetAppLocaleAsBCP47(locale);
+  return IsLocaleRTL(locale);
+}
+
+NS_IMETHODIMP
+LocaleService::Observe(nsISupports* aSubject, const char* aTopic,
+                       const char16_t* aData) {
+  MOZ_ASSERT(mIsServer, "This should only be called in the server mode.");
+
+  if (!strcmp(aTopic, INTL_SYSTEM_LOCALES_CHANGED)) {
+    RequestedLocalesChanged();
+    WebExposedLocalesChanged();
+  } else if (!strcmp(aTopic, NS_XPCOM_SHUTDOWN_OBSERVER_ID)) {
+    RemoveObservers();
+  } else {
+    NS_ConvertUTF16toUTF8 pref(aData);
+    // At the moment the only thing we're observing are settings indicating
+    // user requested locales.
+    if (pref.EqualsLiteral(REQUESTED_LOCALES_PREF)) {
+      RequestedLocalesChanged();
+    } else if (pref.EqualsLiteral(WEB_EXPOSED_LOCALES_PREF)) {
+      WebExposedLocalesChanged();
+    } else if (pref.EqualsLiteral(PSEUDO_LOCALE_PREF)) {
+      NotifyAppLocaleChanged();
+    }
+  }
+
+  return NS_OK;
+}
+
+bool LocaleService::LanguagesMatch(const nsACString& aRequested,
+                                   const nsACString& aAvailable) {
+  Locale requested;
+  auto requestedResult = LocaleParser::TryParse(aRequested, requested);
+  Locale available;
+  auto availableResult = LocaleParser::TryParse(aAvailable, available);
+
+  if (requestedResult.isErr() || availableResult.isErr()) {
+    return false;
+  }
+
+  if (requested.Canonicalize().isErr() || available.Canonicalize().isErr()) {
+    return false;
+  }
+
+  return requested.Language().Span() == available.Language().Span();
+}
+
+bool LocaleService::IsServer() { return mIsServer; }
+
+static bool GetGREFileContents(const char* aFilePath, nsCString* aOutString) {
+  // Look for the requested file in omnijar.
+  RefPtr<nsZipArchive> zip = Omnijar::GetReader(Omnijar::GRE);
+  if (zip) {
+    nsZipItemPtr<char> item(zip, aFilePath);
+    if (!item) {
+      return false;
+    }
+    aOutString->Assign(item.Buffer(), item.Length());
+    return true;
+  }
+
+  // If we didn't have an omnijar (i.e. we're running a non-packaged
+  // build), then look in the GRE directory.
+  nsCOMPtr<nsIFile> path;
+  if (NS_FAILED(nsDirectoryService::gService->Get(
+          NS_GRE_DIR, NS_GET_IID(nsIFile), getter_AddRefs(path)))) {
+    return false;
+  }
+
+  path->AppendRelativeNativePath(nsDependentCString(aFilePath));
+  bool result;
+  if (NS_FAILED(path->IsFile(&result)) || !result ||
+      NS_FAILED(path->IsReadable(&result)) || !result) {
+    return false;
+  }
+
+  // This is a small file, only used once, so it's not worth doing some fancy
+  // off-main-thread file I/O or whatever. Just read it.
+  FILE* fp;
+  if (NS_FAILED(path->OpenANSIFileDesc("r", &fp)) || !fp) {
+    return false;
+  }
+
+  fseek(fp, 0, SEEK_END);
+  long len = ftell(fp);
+  rewind(fp);
+  aOutString->SetLength(len);
+  size_t cc = fread(aOutString->BeginWriting(), 1, len, fp);
+
+  fclose(fp);
+
+  return cc == size_t(len);
+}
+
+void LocaleService::InitPackagedLocales() {
+  MOZ_ASSERT(mPackagedLocales.IsEmpty());
+
+  nsAutoCString localesString;
+  if (GetGREFileContents("res/multilocale.txt", &localesString)) {
+    localesString.Trim(" \t\n\r");
+    // This should never be empty in a correctly-built product.
+    MOZ_ASSERT(!localesString.IsEmpty());
+    SplitLocaleListStringIntoArray(localesString, mPackagedLocales);
+  }
+
+  // Last resort in case of broken build
+  if (mPackagedLocales.IsEmpty()) {
+    nsAutoCString defaultLocale;
+    GetDefaultLocale(defaultLocale);
+    mPackagedLocales.AppendElement(defaultLocale);
+  }
+}
+
+/**
+ * mozILocaleService methods
+ */
+
+NS_IMETHODIMP
+LocaleService::GetDefaultLocale(nsACString& aRetVal) {
+  // We don't allow this to change during a session (it's set at build/package
+  // time), so we cache the result the first time we're called.
+  if (mDefaultLocale.IsEmpty()) {
+    nsAutoCString locale;
+    // Try to get the package locale from update.locale in omnijar. If the
+    // update.locale file is not found, item.len will remain 0 and we'll
+    // just use our hard-coded default below.
+    GetGREFileContents("update.locale", &locale);
+    locale.Trim(" \t\n\r");
+#ifdef MOZ_UPDATER
+    // This should never be empty.
+    MOZ_ASSERT(!locale.IsEmpty());
+#endif
+    if (CanonicalizeLanguageId(locale)) {
+      mDefaultLocale.Assign(locale);
+    }
+
+    // Hard-coded fallback to allow us to survive even if update.locale was
+    // missing/broken in some way.
+    if (mDefaultLocale.IsEmpty()) {
+      GetLastFallbackLocale(mDefaultLocale);
+    }
+  }
+
+  aRetVal = mDefaultLocale;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetLastFallbackLocale(nsACString& aRetVal) {
+  aRetVal.AssignLiteral("en-US");
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetAppLocalesAsLangTags(nsTArray<nsCString>& aRetVal) {
+  if (mAppLocales.IsEmpty()) {
+    NegotiateAppLocales(mAppLocales);
+  }
+  for (uint32_t i = 0; i < mAppLocales.Length(); i++) {
+    nsAutoCString locale(mAppLocales[i]);
+    if (locale.LowerCaseEqualsASCII("ja-jp-macos")) {
+      aRetVal.AppendElement("ja-JP-mac");
+    } else {
+      aRetVal.AppendElement(locale);
+    }
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetAppLocalesAsBCP47(nsTArray<nsCString>& aRetVal) {
+  if (mAppLocales.IsEmpty()) {
+    NegotiateAppLocales(mAppLocales);
+  }
+  aRetVal = mAppLocales.Clone();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetAppLocaleAsLangTag(nsACString& aRetVal) {
+  AutoTArray<nsCString, 32> locales;
+  GetAppLocalesAsLangTags(locales);
+
+  aRetVal = locales[0];
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetAppLocaleAsBCP47(nsACString& aRetVal) {
+  if (mAppLocales.IsEmpty()) {
+    NegotiateAppLocales(mAppLocales);
+  }
+  aRetVal = mAppLocales[0];
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetRegionalPrefsLocales(nsTArray<nsCString>& aRetVal) {
+  bool useOSLocales =
+      Preferences::GetBool("intl.regional_prefs.use_os_locales", false);
+
+  // If the user specified that they want to use OS Regional Preferences
+  // locales, try to retrieve them and use.
+  if (useOSLocales) {
+    if (NS_SUCCEEDED(
+            OSPreferences::GetInstance()->GetRegionalPrefsLocales(aRetVal))) {
+      return NS_OK;
+    }
+
+    // If we fail to retrieve them, return the app locales.
+    GetAppLocalesAsBCP47(aRetVal);
+    return NS_OK;
+  }
+
+  // Otherwise, fetch OS Regional Preferences locales and compare the first one
+  // to the app locale. If the language subtag matches, we can safely use
+  // the OS Regional Preferences locale.
+  //
+  // This facilitates scenarios such as Firefox in "en-US" and User sets
+  // regional prefs to "en-GB".
+  nsAutoCString appLocale;
+  AutoTArray<nsCString, 10> regionalPrefsLocales;
+  LocaleService::GetInstance()->GetAppLocaleAsBCP47(appLocale);
+
+  if (NS_FAILED(OSPreferences::GetInstance()->GetRegionalPrefsLocales(
+          regionalPrefsLocales))) {
+    GetAppLocalesAsBCP47(aRetVal);
+    return NS_OK;
+  }
+
+  if (LocaleService::LanguagesMatch(appLocale, regionalPrefsLocales[0])) {
+    aRetVal = regionalPrefsLocales.Clone();
+    return NS_OK;
+  }
+
+  // Otherwise use the app locales.
+  GetAppLocalesAsBCP47(aRetVal);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetWebExposedLocales(nsTArray<nsCString>& aRetVal) {
+  if (StaticPrefs::privacy_spoof_english() == 2) {
+    aRetVal = nsTArray<nsCString>({"en-US"_ns});
+    return NS_OK;
+  }
+
+  if (!mWebExposedLocales.IsEmpty()) {
+    aRetVal = mWebExposedLocales.Clone();
+    return NS_OK;
+  }
+
+  return GetRegionalPrefsLocales(aRetVal);
+}
+
+NS_IMETHODIMP
+LocaleService::NegotiateLanguages(const nsTArray<nsCString>& aRequested,
+                                  const nsTArray<nsCString>& aAvailable,
+                                  const nsACString& aDefaultLocale,
+                                  int32_t aStrategy,
+                                  nsTArray<nsCString>& aRetVal) {
+  if (aStrategy < 0 || aStrategy > 2) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+#ifdef DEBUG
+  Locale parsedLocale;
+  auto result = LocaleParser::TryParse(aDefaultLocale, parsedLocale);
+
+  MOZ_ASSERT(
+      aDefaultLocale.IsEmpty() || result.isOk(),
+      "If specified, default locale must be a well-formed BCP47 language tag.");
+#endif
+
+  if (aStrategy == kLangNegStrategyLookup && aDefaultLocale.IsEmpty()) {
+    NS_WARNING(
+        "Default locale should be specified when using lookup strategy.");
+  }
+
+  NegotiationStrategy strategy;
+  switch (aStrategy) {
+    case kLangNegStrategyFiltering:
+      strategy = NegotiationStrategy::Filtering;
+      break;
+    case kLangNegStrategyMatching:
+      strategy = NegotiationStrategy::Matching;
+      break;
+    case kLangNegStrategyLookup:
+      strategy = NegotiationStrategy::Lookup;
+      break;
+  }
+
+  fluent_langneg_negotiate_languages(&aRequested, &aAvailable, &aDefaultLocale,
+                                     strategy, &aRetVal);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetRequestedLocales(nsTArray<nsCString>& aRetVal) {
+  if (mRequestedLocales.IsEmpty()) {
+    ReadRequestedLocales(mRequestedLocales);
+  }
+
+  aRetVal = mRequestedLocales.Clone();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetRequestedLocale(nsACString& aRetVal) {
+  if (mRequestedLocales.IsEmpty()) {
+    ReadRequestedLocales(mRequestedLocales);
+  }
+
+  if (mRequestedLocales.Length() > 0) {
+    aRetVal = mRequestedLocales[0];
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::SetRequestedLocales(const nsTArray<nsCString>& aRequested) {
+  MOZ_ASSERT(mIsServer, "This should only be called in the server mode.");
+  if (!mIsServer) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  nsAutoCString str;
+
+  for (auto& req : aRequested) {
+    nsAutoCString locale(req);
+    if (!CanonicalizeLanguageId(locale)) {
+      NS_ERROR("Invalid language tag provided to SetRequestedLocales!");
+      return NS_ERROR_INVALID_ARG;
+    }
+
+    if (!str.IsEmpty()) {
+      str.AppendLiteral(",");
+    }
+    str.Append(locale);
+  }
+  Preferences::SetCString(REQUESTED_LOCALES_PREF, str);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetAvailableLocales(nsTArray<nsCString>& aRetVal) {
+  MOZ_ASSERT(mIsServer, "This should only be called in the server mode.");
+  if (!mIsServer) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  if (mAvailableLocales.IsEmpty()) {
+    // If there are no available locales set, it means that L10nRegistry
+    // did not register its locale pool yet. The best course of action
+    // is to use packaged locales until that happens.
+    GetPackagedLocales(mAvailableLocales);
+  }
+
+  aRetVal = mAvailableLocales.Clone();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetIsAppLocaleRTL(bool* aRetVal) {
+  (*aRetVal) = IsAppLocaleRTL();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::SetAvailableLocales(const nsTArray<nsCString>& aAvailable) {
+  MOZ_ASSERT(mIsServer, "This should only be called in the server mode.");
+  if (!mIsServer) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  nsTArray<nsCString> newLocales;
+
+  for (auto& avail : aAvailable) {
+    nsAutoCString locale(avail);
+    if (!CanonicalizeLanguageId(locale)) {
+      NS_ERROR("Invalid language tag provided to SetAvailableLocales!");
+      return NS_ERROR_INVALID_ARG;
+    }
+    newLocales.AppendElement(locale);
+  }
+
+  if (newLocales != mAvailableLocales) {
+    mAvailableLocales = std::move(newLocales);
+    LocalesChanged();
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+LocaleService::GetPackagedLocales(nsTArray<nsCString>& aRetVal) {
+  if (mPackagedLocales.IsEmpty()) {
+    InitPackagedLocales();
+  }
+  aRetVal = mPackagedLocales.Clone();
+  return NS_OK;
+}
diff --git a/intl/locale/LocaleService.h b/intl/locale/LocaleService.h
new file mode 100644
index 0000000000..f5b2c8dadc
--- /dev/null
+++ b/intl/locale/LocaleService.h
@@ -0,0 +1,232 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode:nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_intl_LocaleService_h__
+#define mozilla_intl_LocaleService_h__
+
+#include "nsIObserver.h"
+#include "nsString.h"
+#include "nsTArray.h"
+#include "nsWeakReference.h"
+#include "MozLocaleBindings.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozILocaleService.h"
+
+namespace mozilla {
+namespace intl {
+
+/**
+ * LocaleService is a manager of language negotiation in Gecko.
+ *
+ * It's intended to be the core place for collecting available and
+ * requested languages and negotiating them to produce a fallback
+ * chain of locales for the application.
+ *
+ * Client / Server
+ *
+ * LocaleService may operate in one of two modes:
+ *
+ *   server
+ *     in the server mode, LocaleService is collecting and negotiating
+ *     languages. It also subscribes to relevant observers.
+ *     There should be at most one server per application instance.
+ *
+ *   client
+ *     in the client mode, LocaleService is not responsible for collecting
+ *     or reacting to any system changes. It still distributes information
+ *     about locales, but internally, it gets information from the server
+ * instance instead of collecting it on its own. This prevents any data
+ * desynchronization and minimizes the cost of running the service.
+ *
+ *   In both modes, all get* methods should work the same way and all
+ *   static methods are available.
+ *
+ *   In the server mode, other components may inform LocaleService about their
+ *   status either via calls to set* methods or via observer events.
+ *   In the client mode, only the process communication should provide data
+ *   to the LocaleService.
+ *
+ *   At the moment desktop apps use the parent process in the server mode, and
+ *   content processes in the client mode.
+ *
+ * Locale / Language
+ *
+ * The terms `Locale ID` and `Language ID` are used slightly differently
+ * by different organizations. Mozilla uses the term `Language ID` to describe
+ * a string that contains information about the language itself, script,
+ * region and variant. For example "en-Latn-US-mac" is a correct Language ID.
+ *
+ * Locale ID contains a Language ID plus a number of extension tags that
+ * contain information that go beyond language inforamation such as
+ * preferred currency, date/time formatting etc.
+ *
+ * An example of a Locale ID is `en-Latn-US-x-hc-h12-ca-gregory`
+ *
+ * At the moment we do not support full extension tag system, but we
+ * try to be specific when naming APIs, so the service is for locales,
+ * but we negotiate between languages etc.
+ */
+class LocaleService final : public mozILocaleService,
+                            public nsIObserver,
+                            public nsSupportsWeakReference {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+  NS_DECL_MOZILOCALESERVICE
+
+  /**
+   * List of available language negotiation strategies.
+   *
+   * See the mozILocaleService.idl for detailed description of the
+   * strategies.
+   */
+  static const int32_t kLangNegStrategyFiltering = 0;
+  static const int32_t kLangNegStrategyMatching = 1;
+  static const int32_t kLangNegStrategyLookup = 2;
+
+  explicit LocaleService(bool aIsServer);
+
+  /**
+   * Create (if necessary) and return a raw pointer to the singleton instance.
+   * Use this accessor in C++ code that just wants to call a method on the
+   * instance, but does not need to hold a reference, as in
+   *    nsAutoCString str;
+   *    LocaleService::GetInstance()->GetAppLocaleAsLangTag(str);
+   */
+  static LocaleService* GetInstance();
+
+  /**
+   * Return an addRef'd pointer to the singleton instance. This is used by the
+   * XPCOM constructor that exists to support usage from JS.
+   */
+  static already_AddRefed<LocaleService> GetInstanceAddRefed() {
+    return RefPtr<LocaleService>(GetInstance()).forget();
+  }
+
+  /**
+   * Canonicalize a Unicode Language Identifier string.
+   *
+   * The operation is:
+   *   * Normalizing casing (`eN-Us-Windows` -> `en-US-windows`)
+   *   * Switching `_` to `-` (`en_US` -> `en-US`)
+   *   * Rejecting invalid identifiers (`e21-X` sets aLocale to `und` and
+   * returns false)
+   *   * Normalizing Mozilla's `ja-JP-mac` to `ja-JP-macos`
+   *   * Cutting off POSIX dot postfix (`en-US.utf8` -> `en-US`)
+   *
+   * This operation should be used on any external input before
+   * it gets used in internal operations.
+   */
+  static bool CanonicalizeLanguageId(nsACString& aLocale) {
+    return ffi::unic_langid_canonicalize(&aLocale);
+  }
+  /**
+   * This method should only be called in the client mode.
+   *
+   * It replaces all the language negotiation and is supposed to be called
+   * in order to bring the client LocaleService in sync with the server
+   * LocaleService.
+   *
+   * Currently, it's called by the IPC code.
+   */
+  void AssignAppLocales(const nsTArray<nsCString>& aAppLocales);
+  void AssignRequestedLocales(const nsTArray<nsCString>& aRequestedLocales);
+
+  /**
+   * Those two functions allow to trigger cache invalidation on one of the
+   * three cached values.
+   *
+   * In most cases, the functions will be called by the observer in
+   * LocaleService itself, but in a couple special cases, we have the
+   * other component call this manually instead of sending a global event.
+   *
+   * If the result differs from the previous list, it will additionally
+   * trigger a corresponding event
+   *
+   * This code should be called only in the server mode..
+   */
+  void RequestedLocalesChanged();
+  void LocalesChanged();
+
+  /**
+   * This function keeps the pref setting updated.
+   */
+  void WebExposedLocalesChanged();
+
+  /**
+   * Returns whether the locale is RTL.
+   */
+  static bool IsLocaleRTL(const nsACString& aLocale);
+
+  /**
+   * Returns whether the current app locale is RTL.
+   *
+   * This method respects this override:
+   *  - `intl.l10n.pseudo`
+   */
+  bool IsAppLocaleRTL();
+
+  static bool LanguagesMatch(const nsACString& aRequested,
+                             const nsACString& aAvailable);
+
+  bool IsServer();
+
+  /**
+   * Create a component from intl/components with the current app's locale. This
+   * is a convenience method for efficient string management with the app
+   * locale.
+   */
+  template <typename T, typename... Args>
+  static Result<UniquePtr<T>, ICUError> TryCreateComponent(Args... args) {
+    // 32 is somewhat arbitrary for the length, but it should fit common
+    // locales, but locales such as the following will be heap allocated:
+    //
+    //  "de-u-ca-gregory-fw-mon-hc-h23-co-phonebk-ka-noignore-kb-false-kc-
+    //    false-kf-false-kh-false-kk-false-kn-false-kr-space-ks-level1-kv-space-cf-
+    //    standard-cu-eur-ms-metric-nu-latn-lb-strict-lw-normal-ss-none-tz-atvie-em-
+    //    default-rg-atzzzz-sd-atat1-va-posix"
+    nsAutoCStringN<32> appLocale;
+    mozilla::intl::LocaleService::GetInstance()->GetAppLocaleAsBCP47(appLocale);
+
+    return T::TryCreate(appLocale.get(), args...);
+  }
+
+  /**
+   * Create a component from intl/components with a given locale, but fallback
+   * to the app locale if it doesn't work.
+   */
+  template <typename T, typename... Args>
+  static Result<UniquePtr<T>, ICUError> TryCreateComponentWithLocale(
+      const char* aLocale, Args... args) {
+    auto result = T::TryCreate(aLocale, args...);
+    if (result.isOk()) {
+      return result;
+    }
+    return TryCreateComponent<T>(args...);
+  }
+
+ private:
+  void NegotiateAppLocales(nsTArray<nsCString>& aRetVal);
+
+  void InitPackagedLocales();
+
+  void RemoveObservers();
+
+  virtual ~LocaleService() = default;
+
+  nsAutoCStringN<16> mDefaultLocale;
+  nsTArray<nsCString> mAppLocales;
+  nsTArray<nsCString> mRequestedLocales;
+  nsTArray<nsCString> mAvailableLocales;
+  nsTArray<nsCString> mPackagedLocales;
+  nsTArray<nsCString> mWebExposedLocales;
+  const bool mIsServer;
+
+  static StaticRefPtr<LocaleService> sInstance;
+};
+}  // namespace intl
+}  // namespace mozilla
+
+#endif /* mozilla_intl_LocaleService_h__ */
diff --git a/intl/locale/MozLocaleBindings.h b/intl/locale/MozLocaleBindings.h
new file mode 100644
index 0000000000..2b71757a44
--- /dev/null
+++ b/intl/locale/MozLocaleBindings.h
@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_intl_locale_MozLocaleBindings_h
+#define mozilla_intl_locale_MozLocaleBindings_h
+
+#include "mozilla/intl/unic_langid_ffi_generated.h"
+#include "mozilla/intl/fluent_langneg_ffi_generated.h"
+
+#include "mozilla/UniquePtr.h"
+
+namespace mozilla {
+
+template <>
+class DefaultDelete<intl::ffi::LanguageIdentifier> {
+ public:
+  void operator()(intl::ffi::LanguageIdentifier* aPtr) const {
+    unic_langid_destroy(aPtr);
+  }
+};
+
+}  // namespace mozilla
+
+#endif
diff --git a/intl/locale/OSPreferences.cpp b/intl/locale/OSPreferences.cpp
new file mode 100644
index 0000000000..b87924b61a
--- /dev/null
+++ b/intl/locale/OSPreferences.cpp
@@ -0,0 +1,585 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This is a shared part of the OSPreferences API implementation.
+ * It defines helper methods and public methods that are calling
+ * platform-specific private methods.
+ */
+
+#include "OSPreferences.h"
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/intl/DateTimePatternGenerator.h"
+#include "mozilla/intl/DateTimeFormat.h"
+#include "mozilla/intl/LocaleService.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/Result.h"
+#include "mozilla/Services.h"
+#include "nsIObserverService.h"
+
+using namespace mozilla::intl;
+
+NS_IMPL_ISUPPORTS(OSPreferences, mozIOSPreferences)
+
+mozilla::StaticRefPtr<OSPreferences> OSPreferences::sInstance;
+
+// Return a new strong reference to the instance, creating it if necessary.
+already_AddRefed<OSPreferences> OSPreferences::GetInstanceAddRefed() {
+  RefPtr<OSPreferences> result = sInstance;
+  if (!result) {
+    MOZ_ASSERT(NS_IsMainThread(),
+               "OSPreferences should be initialized on main thread!");
+    if (!NS_IsMainThread()) {
+      return nullptr;
+    }
+    sInstance = new OSPreferences();
+    result = sInstance;
+
+    DebugOnly<nsresult> rv = Preferences::RegisterPrefixCallback(
+        PreferenceChanged, "intl.date_time.pattern_override");
+    MOZ_ASSERT(NS_SUCCEEDED(rv), "Adding observers failed.");
+
+    ClearOnShutdown(&sInstance);
+  }
+  return result.forget();
+}
+
+// Return a raw pointer to the instance: not for off-main-thread use,
+// because ClearOnShutdown means it could go away unexpectedly.
+OSPreferences* OSPreferences::GetInstance() {
+  MOZ_ASSERT(NS_IsMainThread());
+  if (!sInstance) {
+    // This will create the static instance; then we just drop the extra
+    // reference.
+    RefPtr<OSPreferences> result = GetInstanceAddRefed();
+  }
+  return sInstance;
+}
+
+void OSPreferences::Refresh() {
+  nsTArray<nsCString> newLocales;
+  ReadSystemLocales(newLocales);
+
+  if (mSystemLocales != newLocales) {
+    mSystemLocales = std::move(newLocales);
+    nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
+    if (obs) {
+      obs->NotifyObservers(nullptr, "intl:system-locales-changed", nullptr);
+    }
+  }
+}
+
+OSPreferences::~OSPreferences() {
+  Preferences::UnregisterPrefixCallback(PreferenceChanged,
+                                        "intl.date_time.pattern_override");
+  RemoveObservers();
+}
+
+/*static*/
+void OSPreferences::PreferenceChanged(const char* aPrefName,
+                                      void* /* aClosure */) {
+  if (sInstance) {
+    sInstance->mPatternCache.Clear();
+  }
+}
+
+/**
+ * This method should be called by every method of OSPreferences that
+ * retrieves a locale id from external source.
+ *
+ * It attempts to retrieve as much of the locale ID as possible, cutting
+ * out bits that are not understood (non-strict behavior of ICU).
+ *
+ * It returns true if the canonicalization was successful.
+ */
+bool OSPreferences::CanonicalizeLanguageTag(nsCString& aLoc) {
+  return LocaleService::CanonicalizeLanguageId(aLoc);
+}
+
+/**
+ * This method retrieves from mozilla::intl the best pattern for a given
+ * date/time style.
+ */
+bool OSPreferences::GetDateTimePatternForStyle(DateTimeFormatStyle aDateStyle,
+                                               DateTimeFormatStyle aTimeStyle,
+                                               const nsACString& aLocale,
+                                               nsACString& aRetVal) {
+  DateTimeFormat::StyleBag style;
+
+  switch (aTimeStyle) {
+    case DateTimeFormatStyle::Short:
+      style.time = Some(DateTimeFormat::Style::Short);
+      break;
+    case DateTimeFormatStyle::Medium:
+      style.time = Some(DateTimeFormat::Style::Medium);
+      break;
+    case DateTimeFormatStyle::Long:
+      style.time = Some(DateTimeFormat::Style::Long);
+      break;
+    case DateTimeFormatStyle::Full:
+      style.time = Some(DateTimeFormat::Style::Full);
+      break;
+    case DateTimeFormatStyle::None:
+    case DateTimeFormatStyle::Invalid:
+      // Do nothing.
+      break;
+  }
+
+  switch (aDateStyle) {
+    case DateTimeFormatStyle::Short:
+      style.date = Some(DateTimeFormat::Style::Short);
+      break;
+    case DateTimeFormatStyle::Medium:
+      style.date = Some(DateTimeFormat::Style::Medium);
+      break;
+    case DateTimeFormatStyle::Long:
+      style.date = Some(DateTimeFormat::Style::Long);
+      break;
+    case DateTimeFormatStyle::Full:
+      style.date = Some(DateTimeFormat::Style::Full);
+      break;
+    case DateTimeFormatStyle::None:
+    case DateTimeFormatStyle::Invalid:
+      // Do nothing.
+      break;
+  }
+
+  nsAutoCString locale;
+  if (aLocale.IsEmpty()) {
+    AutoTArray<nsCString, 10> regionalPrefsLocales;
+    LocaleService::GetInstance()->GetRegionalPrefsLocales(regionalPrefsLocales);
+    locale.Assign(regionalPrefsLocales[0]);
+  } else {
+    locale.Assign(aLocale);
+  }
+
+  auto genResult =
+      DateTimePatternGenerator::TryCreate(PromiseFlatCString(aLocale).get());
+  if (genResult.isErr()) {
+    return false;
+  }
+  auto generator = genResult.unwrap();
+
+  auto dfResult = DateTimeFormat::TryCreateFromStyle(
+      MakeStringSpan(locale.get()), style, generator.get(), Nothing());
+  if (dfResult.isErr()) {
+    return false;
+  }
+  auto df = dfResult.unwrap();
+
+  DateTimeFormat::PatternVector pattern;
+  auto patternResult = df->GetPattern(pattern);
+  if (patternResult.isErr()) {
+    return false;
+  }
+
+  aRetVal = NS_ConvertUTF16toUTF8(pattern.begin(), pattern.length());
+  return true;
+}
+
+/**
+ * This method retrieves from mozilla::intl the best skeleton for a given
+ * date/time style.
+ *
+ * This is useful for cases where an OS does not provide its own patterns,
+ * but provide ability to customize the skeleton, like alter hourCycle setting.
+ *
+ * The returned value is a skeleton that matches the styles.
+ */
+bool OSPreferences::GetDateTimeSkeletonForStyle(DateTimeFormatStyle aDateStyle,
+                                                DateTimeFormatStyle aTimeStyle,
+                                                const nsACString& aLocale,
+                                                nsACString& aRetVal) {
+  nsAutoCString pattern;
+  if (!GetDateTimePatternForStyle(aDateStyle, aTimeStyle, aLocale, pattern)) {
+    return false;
+  }
+
+  auto genResult =
+      DateTimePatternGenerator::TryCreate(PromiseFlatCString(aLocale).get());
+  if (genResult.isErr()) {
+    return false;
+  }
+
+  nsAutoString patternAsUtf16 = NS_ConvertUTF8toUTF16(pattern);
+  DateTimeFormat::SkeletonVector skeleton;
+  auto generator = genResult.unwrap();
+  auto skeletonResult = generator->GetSkeleton(patternAsUtf16, skeleton);
+  if (skeletonResult.isErr()) {
+    return false;
+  }
+
+  aRetVal = NS_ConvertUTF16toUTF8(skeleton.begin(), skeleton.length());
+  return true;
+}
+
+/**
+ * This method checks for preferences that override the defaults
+ */
+bool OSPreferences::OverrideDateTimePattern(DateTimeFormatStyle aDateStyle,
+                                            DateTimeFormatStyle aTimeStyle,
+                                            const nsACString& aLocale,
+                                            nsACString& aRetVal) {
+  const auto PrefToMaybeString = [](const char* pref) -> Maybe<nsAutoCString> {
+    nsAutoCString value;
+    nsresult nr = Preferences::GetCString(pref, value);
+    if (NS_FAILED(nr) || value.IsEmpty()) {
+      return Nothing();
+    }
+    return Some(std::move(value));
+  };
+
+  Maybe<nsAutoCString> timeSkeleton;
+  switch (aTimeStyle) {
+    case DateTimeFormatStyle::Short:
+      timeSkeleton =
+          PrefToMaybeString("intl.date_time.pattern_override.time_short");
+      break;
+    case DateTimeFormatStyle::Medium:
+      timeSkeleton =
+          PrefToMaybeString("intl.date_time.pattern_override.time_medium");
+      break;
+    case DateTimeFormatStyle::Long:
+      timeSkeleton =
+          PrefToMaybeString("intl.date_time.pattern_override.time_long");
+      break;
+    case DateTimeFormatStyle::Full:
+      timeSkeleton =
+          PrefToMaybeString("intl.date_time.pattern_override.time_full");
+      break;
+    default:
+      break;
+  }
+
+  Maybe<nsAutoCString> dateSkeleton;
+  switch (aDateStyle) {
+    case DateTimeFormatStyle::Short:
+      dateSkeleton =
+          PrefToMaybeString("intl.date_time.pattern_override.date_short");
+      break;
+    case DateTimeFormatStyle::Medium:
+      dateSkeleton =
+          PrefToMaybeString("intl.date_time.pattern_override.date_medium");
+      break;
+    case DateTimeFormatStyle::Long:
+      dateSkeleton =
+          PrefToMaybeString("intl.date_time.pattern_override.date_long");
+      break;
+    case DateTimeFormatStyle::Full:
+      dateSkeleton =
+          PrefToMaybeString("intl.date_time.pattern_override.date_full");
+      break;
+    default:
+      break;
+  }
+
+  nsAutoCString locale;
+  if (aLocale.IsEmpty()) {
+    AutoTArray<nsCString, 10> regionalPrefsLocales;
+    LocaleService::GetInstance()->GetRegionalPrefsLocales(regionalPrefsLocales);
+    locale.Assign(regionalPrefsLocales[0]);
+  } else {
+    locale.Assign(aLocale);
+  }
+
+  const auto FillConnectorPattern = [&locale](
+                                        const nsAutoCString& datePattern,
+                                        const nsAutoCString& timePattern) {
+    nsAutoCString pattern;
+    GetDateTimeConnectorPattern(nsDependentCString(locale.get()), pattern);
+    int32_t index = pattern.Find("{1}");
+    if (index != kNotFound) {
+      pattern.Replace(index, 3, datePattern);
+    }
+    index = pattern.Find("{0}");
+    if (index != kNotFound) {
+      pattern.Replace(index, 3, timePattern);
+    }
+    return pattern;
+  };
+
+  if (timeSkeleton && dateSkeleton) {
+    aRetVal.Assign(FillConnectorPattern(*dateSkeleton, *timeSkeleton));
+  } else if (timeSkeleton) {
+    if (aDateStyle != DateTimeFormatStyle::None) {
+      nsAutoCString pattern;
+      if (!ReadDateTimePattern(aDateStyle, DateTimeFormatStyle::None, aLocale,
+                               pattern) &&
+          !GetDateTimePatternForStyle(aDateStyle, DateTimeFormatStyle::None,
+                                      aLocale, pattern)) {
+        return false;
+      }
+      aRetVal.Assign(FillConnectorPattern(pattern, *timeSkeleton));
+    } else {
+      aRetVal.Assign(*timeSkeleton);
+    }
+  } else if (dateSkeleton) {
+    if (aTimeStyle != DateTimeFormatStyle::None) {
+      nsAutoCString pattern;
+      if (!ReadDateTimePattern(DateTimeFormatStyle::None, aTimeStyle, aLocale,
+                               pattern) &&
+          !GetDateTimePatternForStyle(DateTimeFormatStyle::None, aTimeStyle,
+                                      aLocale, pattern)) {
+        return false;
+      }
+      aRetVal.Assign(FillConnectorPattern(*dateSkeleton, pattern));
+    } else {
+      aRetVal.Assign(*dateSkeleton);
+    }
+  } else {
+    return false;
+  }
+
+  return true;
+}
+
+/**
+ * This function is a counterpart to GetDateTimeSkeletonForStyle.
+ *
+ * It takes a skeleton and returns the best available pattern for a given locale
+ * that represents the provided skeleton.
+ *
+ * For example:
+ * "Hm" skeleton for "en-US" will return "H:m"
+ */
+bool OSPreferences::GetPatternForSkeleton(const nsACString& aSkeleton,
+                                          const nsACString& aLocale,
+                                          nsACString& aRetVal) {
+  aRetVal.Truncate();
+
+  auto genResult =
+      DateTimePatternGenerator::TryCreate(PromiseFlatCString(aLocale).get());
+  if (genResult.isErr()) {
+    return false;
+  }
+
+  nsAutoString skeletonAsUtf16 = NS_ConvertUTF8toUTF16(aSkeleton);
+  DateTimeFormat::PatternVector pattern;
+  auto generator = genResult.unwrap();
+  auto patternResult = generator->GetBestPattern(skeletonAsUtf16, pattern);
+  if (patternResult.isErr()) {
+    return false;
+  }
+
+  aRetVal = NS_ConvertUTF16toUTF8(pattern.begin(), pattern.length());
+  return true;
+}
+
+/**
+ * This function returns a pattern that should be used to join date and time
+ * patterns into a single date/time pattern string.
+ *
+ * It's useful for OSes that do not provide an API to retrieve such combined
+ * pattern.
+ *
+ * An example output is "{1}, {0}".
+ */
+bool OSPreferences::GetDateTimeConnectorPattern(const nsACString& aLocale,
+                                                nsACString& aRetVal) {
+  // Check for a valid override pref and use that if present.
+  nsAutoCString value;
+  nsresult nr = Preferences::GetCString(
+      "intl.date_time.pattern_override.connector_short", value);
+  if (NS_SUCCEEDED(nr) && value.Find("{0}") != kNotFound &&
+      value.Find("{1}") != kNotFound) {
+    aRetVal = std::move(value);
+    return true;
+  }
+
+  auto genResult =
+      DateTimePatternGenerator::TryCreate(PromiseFlatCString(aLocale).get());
+  if (genResult.isErr()) {
+    return false;
+  }
+
+  auto generator = genResult.unwrap();
+  Span<const char16_t> result = generator->GetPlaceholderPattern();
+  aRetVal = NS_ConvertUTF16toUTF8(result.data(), result.size());
+  return true;
+}
+
+/**
+ * mozIOSPreferences methods
+ */
+NS_IMETHODIMP
+OSPreferences::GetSystemLocales(nsTArray<nsCString>& aRetVal) {
+  if (!mSystemLocales.IsEmpty()) {
+    aRetVal = mSystemLocales.Clone();
+    return NS_OK;
+  }
+
+  if (ReadSystemLocales(aRetVal)) {
+    mSystemLocales = aRetVal.Clone();
+    return NS_OK;
+  }
+
+  // If we failed to get the system locale, we still need
+  // to return something because there are tests out there that
+  // depend on system locale to be set.
+  aRetVal.AppendElement("en-US"_ns);
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+OSPreferences::GetSystemLocale(nsACString& aRetVal) {
+  if (!mSystemLocales.IsEmpty()) {
+    aRetVal = mSystemLocales[0];
+  } else {
+    AutoTArray<nsCString, 10> locales;
+    GetSystemLocales(locales);
+    if (!locales.IsEmpty()) {
+      aRetVal = locales[0];
+    }
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+OSPreferences::GetRegionalPrefsLocales(nsTArray<nsCString>& aRetVal) {
+  if (!mRegionalPrefsLocales.IsEmpty()) {
+    aRetVal = mRegionalPrefsLocales.Clone();
+    return NS_OK;
+  }
+
+  if (ReadRegionalPrefsLocales(aRetVal)) {
+    mRegionalPrefsLocales = aRetVal.Clone();
+    return NS_OK;
+  }
+
+  // If we failed to read regional prefs locales,
+  // use system locales as last fallback.
+  return GetSystemLocales(aRetVal);
+}
+
+static OSPreferences::DateTimeFormatStyle ToDateTimeFormatStyle(
+    int32_t aTimeFormat) {
+  switch (aTimeFormat) {
+    // See mozIOSPreferences.idl for the integer values here.
+    case 0:
+      return OSPreferences::DateTimeFormatStyle::None;
+    case 1:
+      return OSPreferences::DateTimeFormatStyle::Short;
+    case 2:
+      return OSPreferences::DateTimeFormatStyle::Medium;
+    case 3:
+      return OSPreferences::DateTimeFormatStyle::Long;
+    case 4:
+      return OSPreferences::DateTimeFormatStyle::Full;
+  }
+  return OSPreferences::DateTimeFormatStyle::Invalid;
+}
+
+NS_IMETHODIMP
+OSPreferences::GetDateTimePattern(int32_t aDateFormatStyle,
+                                  int32_t aTimeFormatStyle,
+                                  const nsACString& aLocale,
+                                  nsACString& aRetVal) {
+  DateTimeFormatStyle dateStyle = ToDateTimeFormatStyle(aDateFormatStyle);
+  if (dateStyle == DateTimeFormatStyle::Invalid) {
+    return NS_ERROR_INVALID_ARG;
+  }
+  DateTimeFormatStyle timeStyle = ToDateTimeFormatStyle(aTimeFormatStyle);
+  if (timeStyle == DateTimeFormatStyle::Invalid) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  // If the user is asking for None on both, date and time style,
+  // let's exit early.
+  if (timeStyle == DateTimeFormatStyle::None &&
+      dateStyle == DateTimeFormatStyle::None) {
+    return NS_OK;
+  }
+
+  // If the locale is not specified, default to first regional prefs locale
+  const nsACString* locale = &aLocale;
+  AutoTArray<nsCString, 10> rpLocales;
+  if (aLocale.IsEmpty()) {
+    LocaleService::GetInstance()->GetRegionalPrefsLocales(rpLocales);
+    MOZ_ASSERT(rpLocales.Length() > 0);
+    locale = &rpLocales[0];
+  }
+
+  // Create a cache key from the locale + style options
+  nsAutoCString key(*locale);
+  key.Append(':');
+  key.AppendInt(aDateFormatStyle);
+  key.Append(':');
+  key.AppendInt(aTimeFormatStyle);
+
+  nsCString pattern;
+  if (mPatternCache.Get(key, &pattern)) {
+    aRetVal = pattern;
+    return NS_OK;
+  }
+
+  if (!OverrideDateTimePattern(dateStyle, timeStyle, *locale, pattern)) {
+    if (!ReadDateTimePattern(dateStyle, timeStyle, *locale, pattern)) {
+      if (!GetDateTimePatternForStyle(dateStyle, timeStyle, *locale, pattern)) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+  }
+
+  if (mPatternCache.Count() == kMaxCachedPatterns) {
+    // Don't allow unlimited cache growth; just throw it away in the case of
+    // pathological behavior where a page keeps requesting different formats
+    // and locales.
+    NS_WARNING("flushing DateTimePattern cache");
+    mPatternCache.Clear();
+  }
+  mPatternCache.InsertOrUpdate(key, pattern);
+
+  aRetVal = pattern;
+  return NS_OK;
+}
+
+void OSPreferences::OverrideSkeletonHourCycle(bool aIs24Hour,
+                                              nsAutoCString& aSkeleton) {
+  if (aIs24Hour) {
+    // If aSkeleton contains 'h' or 'K', replace with 'H' or 'k' respectively,
+    // and delete 'a' if present.
+    if (aSkeleton.FindChar('h') == -1 && aSkeleton.FindChar('K') == -1) {
+      return;
+    }
+    for (int32_t i = 0; i < int32_t(aSkeleton.Length()); ++i) {
+      switch (aSkeleton[i]) {
+        case 'a':
+          aSkeleton.Cut(i, 1);
+          --i;
+          break;
+        case 'h':
+          aSkeleton.SetCharAt('H', i);
+          break;
+        case 'K':
+          aSkeleton.SetCharAt('k', i);
+          break;
+      }
+    }
+  } else {
+    // If skeleton contains 'H' or 'k', replace with 'h' or 'K' respectively,
+    // and add 'a' unless already present.
+    if (aSkeleton.FindChar('H') == -1 && aSkeleton.FindChar('k') == -1) {
+      return;
+    }
+    bool foundA = false;
+    for (size_t i = 0; i < aSkeleton.Length(); ++i) {
+      switch (aSkeleton[i]) {
+        case 'a':
+          foundA = true;
+          break;
+        case 'H':
+          aSkeleton.SetCharAt('h', i);
+          break;
+        case 'k':
+          aSkeleton.SetCharAt('K', i);
+          break;
+      }
+    }
+    if (!foundA) {
+      aSkeleton.Append(char16_t('a'));
+    }
+  }
+}
diff --git a/intl/locale/OSPreferences.h b/intl/locale/OSPreferences.h
new file mode 100644
index 0000000000..e8d95f823b
--- /dev/null
+++ b/intl/locale/OSPreferences.h
@@ -0,0 +1,185 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_intl_IntlOSPreferences_h__
+#define mozilla_intl_IntlOSPreferences_h__
+
+#include "mozilla/StaticPtr.h"
+#include "nsTHashMap.h"
+#include "nsString.h"
+#include "nsTArray.h"
+
+#include "mozIOSPreferences.h"
+
+namespace mozilla {
+namespace intl {
+
+/**
+ * OSPreferences API provides a set of methods for retrieving information from
+ * the host environment on topics such as:
+ *   - Internationalization
+ *   - Localization
+ *   - Regional preferences
+ *
+ * The API is meant to remain as simple as possible, relaying information from
+ * the host environment to the user without too much logic.
+ *
+ * Saying that, there are two exceptions to that paradigm.
+ *
+ * First one is normalization. We do intend to translate host environment
+ * concepts to unified Intl/L10n vocabulary used by Mozilla.
+ * That means that we will format locale IDs, timezone names, currencies etc.
+ * into a chosen format.
+ *
+ * Second is caching. This API does cache values and where possible will
+ * hook into the environment for some event-driven cache invalidation.
+ *
+ * This means that on platforms that do not support a mechanism to
+ * notify apps about changes, new OS-level settings may not be reflected
+ * in the app until it is relaunched.
+ */
+class OSPreferences : public mozIOSPreferences {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_MOZIOSPREFERENCES
+
+  enum class DateTimeFormatStyle {
+    Invalid = -1,
+    None,
+    Short,   // e.g. time: HH:mm, date: Y/m/d
+    Medium,  // likely same as Short
+    Long,    // e.g. time: including seconds, date: including weekday
+    Full     // e.g. time: with timezone, date: with long weekday, month
+  };
+
+  /**
+   * Constructor, to do any necessary initialization such as registering for
+   * notifications from the system when prefs are modified.
+   */
+  OSPreferences();
+
+  /**
+   * Create (if necessary) and return a raw pointer to the singleton instance.
+   * Use this accessor in C++ code that just wants to call a method on the
+   * instance, but does not need to hold a reference, as in
+   *    nsAutoCString str;
+   *    OSPreferences::GetInstance()->GetSystemLocale(str);
+   *
+   * NOTE that this is not safe for off-main-thread use, because it is possible
+   * that XPCOM shutdown on the main thread could invalidate it at any moment!
+   */
+  static OSPreferences* GetInstance();
+
+  /**
+   * Return an addRef'd pointer to the singleton instance. This is used by the
+   * XPCOM constructor that exists to support usage from JS.
+   */
+  static already_AddRefed<OSPreferences> GetInstanceAddRefed();
+
+  static bool GetPatternForSkeleton(const nsACString& aSkeleton,
+                                    const nsACString& aLocale,
+                                    nsACString& aRetVal);
+
+  static bool GetDateTimeConnectorPattern(const nsACString& aLocale,
+                                          nsACString& aRetVal);
+
+  /**
+   * Triggers a refresh of retrieving data from host environment.
+   *
+   * If the result differs from the previous list, it will additionally
+   * trigger global events for changed values:
+   *
+   *  * SystemLocales: "intl:system-locales-changed"
+   *
+   * This method should not be called from anywhere except of per-platform
+   * hooks into OS events.
+   */
+  void Refresh();
+
+ protected:
+  nsTArray<nsCString> mSystemLocales;
+  nsTArray<nsCString> mRegionalPrefsLocales;
+
+  const size_t kMaxCachedPatterns = 15;
+  nsTHashMap<nsCStringHashKey, nsCString> mPatternCache;
+
+ private:
+  virtual ~OSPreferences();
+
+  static StaticRefPtr<OSPreferences> sInstance;
+
+  static bool CanonicalizeLanguageTag(nsCString& aLoc);
+
+  /**
+   * Helper methods to get formats from ICU; these will return false
+   * in case of error, in which case the caller cannot rely on aRetVal.
+   */
+  bool GetDateTimePatternForStyle(DateTimeFormatStyle aDateStyle,
+                                  DateTimeFormatStyle aTimeStyle,
+                                  const nsACString& aLocale,
+                                  nsACString& aRetVal);
+
+  bool GetDateTimeSkeletonForStyle(DateTimeFormatStyle aDateStyle,
+                                   DateTimeFormatStyle aTimeStyle,
+                                   const nsACString& aLocale,
+                                   nsACString& aRetVal);
+
+  bool OverrideDateTimePattern(DateTimeFormatStyle aDateStyle,
+                               DateTimeFormatStyle aTimeStyle,
+                               const nsACString& aLocale, nsACString& aRetVal);
+
+  /**
+   * This is a host environment specific method that will be implemented
+   * separately for each platform.
+   *
+   * It is only called when the cache is empty or invalidated.
+   *
+   * The return value indicates whether the function successfully
+   * resolved at least one locale.
+   */
+  bool ReadSystemLocales(nsTArray<nsCString>& aRetVal);
+
+  bool ReadRegionalPrefsLocales(nsTArray<nsCString>& aRetVal);
+
+  /**
+   * This is a host environment specific method that will be implemented
+   * separately for each platform.
+   *
+   * It is `best-effort` kind of API that attempts to construct the best
+   * possible date/time pattern for the given styles and locales.
+   *
+   * In case we fail to, or don't know how to retrieve the pattern in a
+   * given environment this function will return false.
+   * Callers should always be prepared to handle that scenario.
+   *
+   * The heuristic may depend on the OS API and HIG guidelines.
+   */
+  bool ReadDateTimePattern(DateTimeFormatStyle aDateFormatStyle,
+                           DateTimeFormatStyle aTimeFormatStyle,
+                           const nsACString& aLocale, nsACString& aRetVal);
+
+  /**
+   * This is called to override the hour cycle in the skeleton based upon
+   * the OS preference for AM/PM or 24 hour display.
+   */
+  void OverrideSkeletonHourCycle(bool aIs24Hour, nsAutoCString& aSkeleton);
+
+  /**
+   * This is called by the destructor to clean up any OS specific observers
+   * that are registered.
+   */
+  void RemoveObservers();
+
+  /**
+   * This is called by the destructor to clean up any OS specific observers
+   * that are registered.
+   */
+  static void PreferenceChanged(const char* aPrefName, void* /* aClosure */);
+};
+
+}  // namespace intl
+}  // namespace mozilla
+
+#endif /* mozilla_intl_IntlOSPreferences_h__ */
diff --git a/intl/locale/Quotes.cpp b/intl/locale/Quotes.cpp
new file mode 100644
index 0000000000..87c47bfa5e
--- /dev/null
+++ b/intl/locale/Quotes.cpp
@@ -0,0 +1,93 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode:nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "Quotes.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/intl/Locale.h"
+#include "nsTHashMap.h"
+#include "nsPrintfCString.h"
+
+using namespace mozilla;
+using namespace mozilla::intl;
+
+namespace {
+struct LangQuotesRec {
+  const char* mLangs;
+  Quotes mQuotes;
+};
+
+#include "cldr-quotes.inc"
+
+static StaticAutoPtr<nsTHashMap<nsCStringHashKey, Quotes>> sQuotesForLang;
+}  // anonymous namespace
+
+namespace mozilla {
+namespace intl {
+
+const Quotes* QuotesForLang(const nsAtom* aLang) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  // On first use, initialize the hashtable from our CLDR-derived data array.
+  if (!sQuotesForLang) {
+    sQuotesForLang = new nsTHashMap<nsCStringHashKey, Quotes>(32);
+    ClearOnShutdown(&sQuotesForLang);
+    for (const auto& i : sLangQuotes) {
+      const char* s = i.mLangs;
+      size_t len;
+      while ((len = strlen(s))) {
+        sQuotesForLang->InsertOrUpdate(nsDependentCString(s, len), i.mQuotes);
+        s += len + 1;
+      }
+    }
+  }
+
+  nsAtomCString langStr(aLang);
+  const Quotes* entry = sQuotesForLang->Lookup(langStr).DataPtrOrNull();
+  if (entry) {
+    // Found an exact match for the requested lang.
+    return entry;
+  }
+
+  // Try parsing lang as a Locale and canonicalizing the subtags, then see if
+  // we can match it with region or script subtags, if present, or just the
+  // primary language tag.
+  Locale loc;
+  auto result = LocaleParser::TryParse(langStr, loc);
+  if (result.isErr()) {
+    return nullptr;
+  }
+  if (loc.Canonicalize().isErr()) {
+    return nullptr;
+  }
+  if (loc.Region().Present()) {
+    nsAutoCString langAndRegion;
+    langAndRegion.Append(loc.Language().Span());
+    langAndRegion.Append('-');
+    langAndRegion.Append(loc.Region().Span());
+    if ((entry = sQuotesForLang->Lookup(langAndRegion).DataPtrOrNull())) {
+      return entry;
+    }
+  }
+  if (loc.Script().Present()) {
+    nsAutoCString langAndScript;
+    langAndScript.Append(loc.Language().Span());
+    langAndScript.Append('-');
+    langAndScript.Append(loc.Script().Span());
+    if ((entry = sQuotesForLang->Lookup(langAndScript).DataPtrOrNull())) {
+      return entry;
+    }
+  }
+  Span<const char> langAsSpan = loc.Language().Span();
+  nsAutoCString lang(langAsSpan.data(), langAsSpan.size());
+  if ((entry = sQuotesForLang->Lookup(lang).DataPtrOrNull())) {
+    return entry;
+  }
+
+  return nullptr;
+}
+
+}  // namespace intl
+}  // namespace mozilla
diff --git a/intl/locale/Quotes.h b/intl/locale/Quotes.h
new file mode 100644
index 0000000000..7bd7d277ca
--- /dev/null
+++ b/intl/locale/Quotes.h
@@ -0,0 +1,35 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode:nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_intl_Quotes_h__
+#define mozilla_intl_Quotes_h__
+
+#include "nsAtom.h"
+
+namespace mozilla {
+namespace intl {
+
+// Currently, all the quotation characters provided by CLDR are single BMP
+// codepoints, so they fit into char16_t fields. If there are ever multi-
+// character strings or non-BMP codepoints in a future version, we'll need
+// to extend this to a larger/more flexible structure, but for now it's
+// deliberately kept simple and lightweight.
+struct Quotes {
+  // Entries in order [open, close, alternativeOpen, alternativeClose]
+  char16_t mChars[4];
+};
+
+/**
+ * Return a pointer to the Quotes record for the given locale (lang attribute),
+ * or nullptr if none available.
+ * The returned value points to a hashtable entry, but will remain valid until
+ * shutdown begins, as the table is not modified after initialization.
+ */
+const Quotes* QuotesForLang(const nsAtom* aLang);
+
+}  // namespace intl
+}  // namespace mozilla
+
+#endif  // mozilla_intl_Quotes_h__
diff --git a/intl/locale/android/OSPreferences_android.cpp b/intl/locale/android/OSPreferences_android.cpp
new file mode 100644
index 0000000000..8ef27c526b
--- /dev/null
+++ b/intl/locale/android/OSPreferences_android.cpp
@@ -0,0 +1,69 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "OSPreferences.h"
+#include "mozilla/Preferences.h"
+
+#include "mozilla/java/GeckoAppShellWrappers.h"
+
+using namespace mozilla::intl;
+
+OSPreferences::OSPreferences() {}
+
+bool OSPreferences::ReadSystemLocales(nsTArray<nsCString>& aLocaleList) {
+  if (!mozilla::jni::IsAvailable()) {
+    return false;
+  }
+
+  // XXX: Notice, this value may be empty on an early read. In that case
+  //     we won't add anything to the return list so that it doesn't get
+  //     cached in mSystemLocales.
+  auto locales = java::GeckoAppShell::GetDefaultLocales();
+  if (locales) {
+    for (size_t i = 0; i < locales->Length(); i++) {
+      jni::String::LocalRef locale = locales->GetElement(i);
+      aLocaleList.AppendElement(locale->ToCString());
+    }
+    return true;
+  }
+  return false;
+}
+
+bool OSPreferences::ReadRegionalPrefsLocales(nsTArray<nsCString>& aLocaleList) {
+  // For now we're just taking System Locales since we don't know of any better
+  // API for regional prefs.
+  return ReadSystemLocales(aLocaleList);
+}
+
+/*
+ * Similar to Gtk, Android does not provide a way to customize or format
+ * date/time patterns, so we're reusing ICU data here, but we do modify it
+ * according to the Android DateFormat is24HourFormat setting.
+ */
+bool OSPreferences::ReadDateTimePattern(DateTimeFormatStyle aDateStyle,
+                                        DateTimeFormatStyle aTimeStyle,
+                                        const nsACString& aLocale,
+                                        nsACString& aRetVal) {
+  if (!mozilla::jni::IsAvailable()) {
+    return false;
+  }
+
+  nsAutoCString skeleton;
+  if (!GetDateTimeSkeletonForStyle(aDateStyle, aTimeStyle, aLocale, skeleton)) {
+    return false;
+  }
+
+  // Customize the skeleton if necessary to reflect user's 12/24hr pref
+  OverrideSkeletonHourCycle(java::GeckoAppShell::GetIs24HourFormat(), skeleton);
+
+  if (!GetPatternForSkeleton(skeleton, aLocale, aRetVal)) {
+    return false;
+  }
+
+  return true;
+}
+
+void OSPreferences::RemoveObservers() {}
diff --git a/intl/locale/android/moz.build b/intl/locale/android/moz.build
new file mode 100644
index 0000000000..b0cb0cc6a5
--- /dev/null
+++ b/intl/locale/android/moz.build
@@ -0,0 +1,13 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+UNIFIED_SOURCES += ["OSPreferences_android.cpp"]
+
+FINAL_LIBRARY = "xul"
+
+LOCAL_INCLUDES += [
+    "..",
+]
diff --git a/intl/locale/cldr-quotes.inc b/intl/locale/cldr-quotes.inc
new file mode 100644
index 0000000000..84b5569308
--- /dev/null
+++ b/intl/locale/cldr-quotes.inc
@@ -0,0 +1,45 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Derived from the Unicode Common Locale Data Repository by cldr-quotes.pl.
+ *
+ * For terms of use, see http://www.unicode.org/copyright.html.
+ */
+
+/*
+ * Created on Wed Oct 26 05:55:58 2022 from CLDR data file cldr-common-42.0.zip.
+ *
+ * * * * * This file contains MACHINE-GENERATED DATA, do not edit! * * * * *
+ *
+ * (generated by intl/locale/cldr-quotes.pl)
+ */
+
+static const LangQuotesRec sLangQuotes[] = {
+  // clang-format off
+  { "af\0ak\0as\0asa\0az\0bem\0bez\0bn\0brx\0ccp\0ceb\0cgg\0chr\0cy\0da\0dav\0dje\0doi\0dz\0ebu\0ee\0en\0es-419\0ff-Adlm\0fil\0fo\0gd\0gl\0gu\0guz\0hi\0id\0jmc\0jv\0kam\0kde\0kea\0khq\0ki\0kln\0km\0kn\0ko\0kok\0ksb\0ku\0lg\0ln\0lo\0lrc\0lu\0luo\0lv\0mas\0mer\0mfe\0mgo\0mi\0ml\0mn\0mr\0ms\0mt\0my\0naq\0nd\0ne\0nus\0nyn\0or\0pa\0pcm\0pis\0rof\0rwk\0saq\0sat\0sbp\0sd\0seh\0ses\0si\0sw\0ta\0te\0teo\0th\0to\0tr\0tt\0twq\0tzm\0uz-Cyrl\0vai\0vi\0vun\0wo\0xh\0xog\0yo\0yue-Hans\0zh\0zu\0", { { 0x201c, 0x201d, 0x2018, 0x2019 } } },
+  { "agq\0ff\0", { { 0x201e, 0x201d, 0x201a, 0x2019 } } },
+  { "am\0az-Cyrl\0fa\0fr-CH\0gsw\0jgo\0kkj\0mzn\0", { { 0xab, 0xbb, 0x2039, 0x203a } } },
+  { "ar\0ur\0", { { 0x201d, 0x201c, 0x2019, 0x2018 } } },
+  { "ast\0bm\0br\0ca\0dyo\0el\0es\0eu\0ewo\0it\0kab\0kk\0mg\0mua\0nnh\0pt-PT\0sc\0sg\0sq\0ti\0", { { 0xab, 0xbb, 0x201c, 0x201d } } },
+  { "bas\0be\0cv\0ky\0ru\0sah\0uk\0", { { 0xab, 0xbb, 0x201e, 0x201c } } },
+  { "bg\0lt\0", { { 0x201e, 0x201c, 0x201e, 0x201c } } },
+  { "bs-Cyrl\0cs\0de\0dsb\0et\0hr\0hsb\0is\0lb\0luy\0mk\0sk\0sl\0", { { 0x201e, 0x201c, 0x201a, 0x2018 } } },
+  { "bs\0", { { 0x201e, 0x201d, 0x2018, 0x2019 } } },
+  { "dua\0ksf\0no\0rw\0", { { 0xab, 0xbb, 0x2018, 0x2019 } } },
+  { "fi\0he\0lag\0rn\0sn\0sv\0", { { 0x201d, 0x201d, 0x2019, 0x2019 } } },
+  { "fr-CA\0", { { 0xab, 0xbb, 0x201d, 0x201c } } },
+  { "fr\0hy\0yav\0", { { 0xab, 0xbb, 0xab, 0xbb } } },
+  { "hu\0", { { 0x201e, 0x201d, 0xbb, 0xab } } },
+  { "ia\0ti-ER\0", { { 0x2018, 0x2019, 0x201c, 0x201d } } },
+  { "ja\0yue\0zh-Hant\0", { { 0x300c, 0x300d, 0x300e, 0x300f } } },
+  { "ka\0", { { 0x201e, 0x201c, 0xab, 0xbb } } },
+  { "nl\0", { { 0x2018, 0x2019, 0x2018, 0x2019 } } },
+  { "nmg\0pl\0ro\0", { { 0x201e, 0x201d, 0xab, 0xbb } } },
+  { "shi\0zgh\0", { { 0xab, 0xbb, 0x201e, 0x201d } } },
+  { "sr\0", { { 0x201e, 0x201c, 0x2018, 0x2018 } } },
+  { "tk\0", { { 0x201c, 0x201d, 0x201c, 0x201d } } },
+  { "uz\0", { { 0x201c, 0x201d, 0x2019, 0x2018 } } },
+  // clang-format on
+};
diff --git a/intl/locale/cldr-quotes.pl b/intl/locale/cldr-quotes.pl
new file mode 100644
index 0000000000..76b91fd5a4
--- /dev/null
+++ b/intl/locale/cldr-quotes.pl
@@ -0,0 +1,108 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# Tool to generate the cldr-quotes.inc file, to be #include'd in Quotes.cpp
+# to provide locale-appropriate opening and closing quote marks.
+
+# To regenerate cldr-quotes.inc for a new CLDR release, download the data file
+# "cldr-common-##.zip" from http://unicode.org/Public/cldr/latest into the
+# current directory, run
+#
+#   perl cldr-quotes.pl <filename>  > cldr-quotes.inc
+#
+# (where <filename> is the downloaded cldr-common-## archive), and
+# then use `hg diff` to check that the result looks sane.
+
+use warnings;
+use strict;
+
+use Encode;
+use IO::Uncompress::Unzip "unzip";
+
+die "Usage: perl cldr-quotes.pl <filename>" unless $#ARGV == 0;
+
+my $filename = $ARGV[0];
+
+my (%langQuotes, %quoteLangs);
+
+my $zip = IO::Uncompress::Unzip->new($filename) ||
+  die "unzip failed: $IO::Uncompress::Unzip::UnzipError\n";
+
+my $status = 1;
+while ($status > 0) {
+  my $name = $zip->getHeaderInfo()->{Name};
+  if ($name =~ m@common/main/([A-Za-z0-9_]+)\.xml@) {
+    my $lang = $1;
+    $lang =~ s/_/-/;
+    while (<$zip>) {
+      $langQuotes{$lang}[0] = $1 if (m!<quotationStart>(.+)<!);
+      $langQuotes{$lang}[1] = $1 if (m!<quotationEnd>(.+)<!);
+      $langQuotes{$lang}[2] = $1 if (m!<alternateQuotationStart>(.+)<!);
+      $langQuotes{$lang}[3] = $1 if (m!<alternateQuotationEnd>(.+)<!);
+    }
+  }
+  $status = $zip->nextStream();
+}
+$zip->close;
+
+foreach my $lang (sort keys %langQuotes) {
+  # We don't actually want to emit anything for the root locale
+  next if $lang eq "root";
+
+  # Inherit any missing entries from the locale's parent
+  my $parent = $lang;
+  while ($parent =~ m/\-/) {
+    # Strip off a trailing subtag to find a parent locale code
+    $parent =~ s/\-[^-]+$//;
+    # Fill in any values available from the parent
+    for (my $i = 0; $i < 4; $i++) {
+      $langQuotes{$lang}[$i] = $langQuotes{$parent}[$i] unless $langQuotes{$lang}[$i];
+    }
+  }
+
+  # Anything still missing is copied from the root locale
+  for (my $i = 0; $i < 4; $i++) {
+    $langQuotes{$lang}[$i] = $langQuotes{"root"}[$i] unless $langQuotes{$lang}[$i];
+  }
+
+  # If the locale ends up the same as its parent, skip
+  next if ($parent ne $lang) && (exists $langQuotes{$parent}) &&
+    (join(",", @{$langQuotes{$lang}}) eq join(",", @{$langQuotes{$parent}}));
+
+  # Create a string with the C source form for the array of 4 quote characters
+  my $quoteChars = join(", ", map { sprintf("0x%x", ord Encode::decode("UTF-8", $_)) } @{$langQuotes{$lang}});
+
+  # Record this locale in the list of those which use this particular set of quotes
+  $quoteLangs{$quoteChars} = [] unless exists $quoteLangs{$quoteChars};
+  push @{$quoteLangs{$quoteChars}}, $lang;
+}
+
+# Output each unique list of quotes, with the string of associated locales
+my $timestamp = gmtime();
+print <<__EOT__;
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Derived from the Unicode Common Locale Data Repository by cldr-quotes.pl.
+ *
+ * For terms of use, see http://www.unicode.org/copyright.html.
+ */
+
+/*
+ * Created on $timestamp from CLDR data file $filename.
+ *
+ * * * * * This file contains MACHINE-GENERATED DATA, do not edit! * * * * *
+ *
+ * (generated by intl/locale/cldr-quotes.pl)
+ */
+
+__EOT__
+
+print "static const LangQuotesRec sLangQuotes[] = {\n";
+print "  // clang-format off\n";
+print sort map { sprintf("  { \"%s\\0\", { { %s } } },\n", join("\\0", sort @{$quoteLangs{$_}}), $_) } (keys %quoteLangs);
+print "  // clang-format on\n";
+print "};\n";
diff --git a/intl/locale/components.conf b/intl/locale/components.conf
new file mode 100644
index 0000000000..2be9966043
--- /dev/null
+++ b/intl/locale/components.conf
@@ -0,0 +1,27 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Classes = [
+    {
+        'name': 'Locale',
+	'js_name': 'locale',
+        'cid': '{92735ff4-6384-4ad6-8508-757010e149ee}',
+        'contract_ids': ['@mozilla.org/intl/localeservice;1'],
+        'interfaces': ['mozILocaleService'],
+        'singleton': True,
+        'type': 'mozilla::intl::LocaleService',
+        'headers': ['mozilla/intl/LocaleService.h'],
+        'constructor': 'mozilla::intl::LocaleService::GetInstanceAddRefed',
+    },
+    {
+        'cid': '{65944815-e9ae-48bd-a2bf-f1108720950c}',
+        'contract_ids': ['@mozilla.org/intl/ospreferences;1'],
+        'singleton': True,
+        'type': 'mozilla::intl::OSPreferences',
+        'headers': ['mozilla/intl/OSPreferences.h'],
+        'constructor': 'mozilla::intl::OSPreferences::GetInstanceAddRefed',
+    },
+]
diff --git a/intl/locale/encodingsgroups.properties b/intl/locale/encodingsgroups.properties
new file mode 100644
index 0000000000..f631bfa64f
--- /dev/null
+++ b/intl/locale/encodingsgroups.properties
@@ -0,0 +1,40 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# x-unicode is assumed for encodings not listed here
+
+Big5=zh-TW
+EUC-JP=ja
+EUC-KR=ko
+gb18030=zh-CN
+GBK=zh-CN
+IBM866=x-cyrillic
+ISO-2022-JP=ja
+ISO-8859-3=x-western
+ISO-8859-4=x-western
+ISO-8859-5=x-cyrillic
+ISO-8859-6=ar
+ISO-8859-7=el
+ISO-8859-8=he
+ISO-8859-8-I=he
+ISO-8859-10=x-western
+ISO-8859-13=x-western
+ISO-8859-14=x-western
+ISO-8859-15=x-western
+ISO-8859-16=x-western
+ISO-8859-2=x-western
+KOI8-R=x-cyrillic
+KOI8-U=x-cyrillic
+Shift_JIS=ja
+windows-1250=x-western
+windows-1251=x-cyrillic
+windows-1252=x-western
+windows-1253=el
+windows-1254=x-western
+windows-1255=he
+windows-1256=ar
+windows-1257=x-western
+windows-1258=x-western
+windows-874=th
+x-mac-cyrillic=x-cyrillic
diff --git a/intl/locale/gtk/OSPreferences_gtk.cpp b/intl/locale/gtk/OSPreferences_gtk.cpp
new file mode 100644
index 0000000000..e7e5d6017d
--- /dev/null
+++ b/intl/locale/gtk/OSPreferences_gtk.cpp
@@ -0,0 +1,120 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <locale.h>
+#include "mozilla/intl/Locale.h"
+#include "OSPreferences.h"
+
+#include "nsServiceManagerUtils.h"
+#include "nsIGSettingsService.h"
+
+using namespace mozilla;
+using namespace mozilla::intl;
+
+OSPreferences::OSPreferences() = default;
+
+bool OSPreferences::ReadSystemLocales(nsTArray<nsCString>& aLocaleList) {
+  MOZ_ASSERT(aLocaleList.IsEmpty());
+
+  nsAutoCString defaultLang(Locale::GetDefaultLocale());
+
+  if (CanonicalizeLanguageTag(defaultLang)) {
+    aLocaleList.AppendElement(defaultLang);
+    return true;
+  }
+  return false;
+}
+
+bool OSPreferences::ReadRegionalPrefsLocales(nsTArray<nsCString>& aLocaleList) {
+  MOZ_ASSERT(aLocaleList.IsEmpty());
+
+  // For now we're just taking the LC_TIME from POSIX environment for all
+  // regional preferences.
+  nsAutoCString localeStr(setlocale(LC_TIME, nullptr));
+
+  if (CanonicalizeLanguageTag(localeStr)) {
+    aLocaleList.AppendElement(localeStr);
+    return true;
+  }
+
+  return false;
+}
+
+/*
+ * This looks up into gtk settings for hourCycle format.
+ *
+ * This works for all GUIs that use gtk settings like Gnome, Elementary etc.
+ *
+ * We're taking the current 12/24h settings irrelevant of the locale, because
+ * in the UI user selects this setting for all locales.
+ */
+static int HourCycle() {
+  nsCOMPtr<nsIGSettingsService> gsettings =
+      do_GetService(NS_GSETTINGSSERVICE_CONTRACTID);
+  if (!gsettings) {
+    return 0;
+  }
+
+  nsCOMPtr<nsIGSettingsCollection> desktop_settings;
+  gsettings->GetCollectionForSchema("org.gnome.desktop.interface"_ns,
+                                    getter_AddRefs(desktop_settings));
+  if (!desktop_settings) {
+    return 0;
+  }
+
+  nsAutoCString result;
+  desktop_settings->GetString("clock-format"_ns, result);
+  if (result == "12h") {
+    return 12;
+  }
+  if (result == "24h") {
+    return 24;
+  }
+  return 0;
+}
+
+/**
+ * Since Gtk does not provide a way to customize or format date/time patterns,
+ * we're reusing ICU data here, but we do modify it according to the only
+ * setting Gtk gives us - hourCycle.
+ *
+ * This means that for gtk we will return a pattern from ICU altered to
+ * represent h12/h24 hour cycle if the user modified the default value.
+ *
+ * In short, this should work like this:
+ *
+ *  * gtk defaults, pl: 24h
+ *  * gtk defaults, en: 12h
+ *
+ *  * gtk 12h, pl: 12h
+ *  * gtk 12h, en: 12h
+ *
+ *  * gtk 24h, pl: 24h
+ *  * gtk 12h, en: 12h
+ */
+bool OSPreferences::ReadDateTimePattern(DateTimeFormatStyle aDateStyle,
+                                        DateTimeFormatStyle aTimeStyle,
+                                        const nsACString& aLocale,
+                                        nsACString& aRetVal) {
+  nsAutoCString skeleton;
+  if (!GetDateTimeSkeletonForStyle(aDateStyle, aTimeStyle, aLocale, skeleton)) {
+    return false;
+  }
+
+  // Customize the skeleton if necessary to reflect user's 12/24hr pref
+  int hourCycle = HourCycle();
+  if (hourCycle == 12 || hourCycle == 24) {
+    OverrideSkeletonHourCycle(hourCycle == 24, skeleton);
+  }
+
+  if (!GetPatternForSkeleton(skeleton, aLocale, aRetVal)) {
+    return false;
+  }
+
+  return true;
+}
+
+void OSPreferences::RemoveObservers() {}
diff --git a/intl/locale/gtk/moz.build b/intl/locale/gtk/moz.build
new file mode 100644
index 0000000000..3b977a9720
--- /dev/null
+++ b/intl/locale/gtk/moz.build
@@ -0,0 +1,15 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+SOURCES += ["OSPreferences_gtk.cpp"]
+
+CXXFLAGS += CONFIG["GLIB_CFLAGS"]
+
+FINAL_LIBRARY = "xul"
+
+LOCAL_INCLUDES += [
+    "..",
+]
diff --git a/intl/locale/language.properties b/intl/locale/language.properties
new file mode 100644
index 0000000000..2f40d20aa5
--- /dev/null
+++ b/intl/locale/language.properties
@@ -0,0 +1,290 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+aa.accept     = true
+ab.accept     = true
+ach.accept    = true
+ae.accept     = true
+af.accept     = true
+ak.accept     = true
+am.accept     = true
+an.accept     = true
+ar.accept     = true
+ar-ae.accept  = true
+ar-bh.accept  = true
+ar-dz.accept  = true
+ar-eg.accept  = true
+ar-iq.accept  = true
+ar-jo.accept  = true
+ar-kw.accept  = true
+ar-lb.accept  = true
+ar-ly.accept  = true
+ar-ma.accept  = true
+ar-om.accept  = true
+ar-qa.accept  = true
+ar-sa.accept  = true
+ar-sy.accept  = true
+ar-tn.accept  = true
+ar-ye.accept  = true
+as.accept     = true
+ast.accept    = true
+av.accept     = true
+ay.accept     = true
+az.accept     = true
+ba.accept     = true
+be.accept     = true
+bg.accept     = true
+bh.accept     = true
+bi.accept     = true
+bm.accept     = true
+bn.accept     = true
+bo.accept     = true
+br.accept     = true
+bs.accept     = true
+ca.accept     = true
+ca-valencia.accept = true
+cak.accept    = true
+ce.accept     = true
+ch.accept     = true
+co.accept     = true
+cr.accept     = true
+crh.accept    = true
+cs.accept     = true
+csb.accept    = true
+cu.accept     = true
+cv.accept     = true
+cy.accept     = true
+da.accept     = true
+de.accept     = true
+de-at.accept  = true
+de-ch.accept  = true
+de-de.accept  = true
+de-li.accept  = true
+de-lu.accept  = true
+dsb.accept    = true
+dv.accept     = true
+dz.accept     = true
+ee.accept     = true
+el.accept     = true
+en.accept     = true
+en-au.accept  = true
+en-bz.accept  = true
+en-ca.accept  = true
+en-gb.accept  = true
+en-ie.accept  = true
+en-jm.accept  = true
+en-nz.accept  = true
+en-ph.accept  = true
+en-tt.accept  = true
+en-us.accept  = true
+en-za.accept  = true
+en-zw.accept  = true
+eo.accept     = true
+es.accept     = true
+es-ar.accept  = true
+es-bo.accept  = true
+es-cl.accept  = true
+es-co.accept  = true
+es-cr.accept  = true
+es-do.accept  = true
+es-ec.accept  = true
+es-es.accept  = true
+es-gt.accept  = true
+es-hn.accept  = true
+es-mx.accept  = true
+es-ni.accept  = true
+es-pa.accept  = true
+es-pe.accept  = true
+es-pr.accept  = true
+es-py.accept  = true
+es-sv.accept  = true
+es-uy.accept  = true
+es-ve.accept  = true
+et.accept     = true
+eu.accept     = true
+fa.accept     = true
+fa-ir.accept  = true
+ff.accept     = true
+fi.accept     = true
+fj.accept     = true
+fo.accept     = true
+fr.accept     = true
+fr-be.accept  = true
+fr-ca.accept  = true
+fr-ch.accept  = true
+fr-fr.accept  = true
+fr-lu.accept  = true
+fr-mc.accept  = true
+fur.accept    = true
+fy.accept     = true
+ga.accept     = true
+gd.accept     = true
+gl.accept     = true
+gn.accept     = true
+gu.accept     = true
+gv.accept     = true
+ha.accept     = true
+haw.accept    = true
+he.accept     = true
+hi.accept     = true
+hil.accept    = true
+ho.accept     = true
+hr.accept     = true
+hsb.accept    = true
+ht.accept     = true
+hu.accept     = true
+hy.accept     = true
+hz.accept     = true
+ia.accept     = true
+id.accept     = true
+ie.accept     = true
+ig.accept     = true
+ii.accept     = true
+ik.accept     = true
+io.accept     = true
+is.accept     = true
+it.accept     = true
+it-ch.accept  = true
+iu.accept     = true
+ja.accept     = true
+jv.accept     = true
+ka.accept     = true
+kab.accept    = true
+kg.accept     = true
+ki.accept     = true
+kk.accept     = true
+kl.accept     = true
+km.accept     = true
+kn.accept     = true
+ko.accept     = true
+ko-kp.accept  = true
+ko-kr.accept  = true
+kok.accept    = true
+kr.accept     = true
+ks.accept     = true
+ku.accept     = true
+kv.accept     = true
+kw.accept     = true
+ky.accept     = true
+la.accept     = true
+lb.accept     = true
+lg.accept     = true
+li.accept     = true
+lij.accept    = true
+ln.accept     = true
+lo.accept     = true
+lt.accept     = true
+ltg.accept    = true
+lu.accept     = true
+lv.accept     = true
+mai.accept    = true
+meh.accept    = true
+mg.accept     = true
+mh.accept     = true
+mi.accept     = true
+mix.accept    = true
+mk.accept     = true
+mk-mk.accept  = true
+ml.accept     = true
+mn.accept     = true
+mr.accept     = true
+ms.accept     = true
+mt.accept     = true
+my.accept     = true
+na.accept     = true
+nb.accept     = true
+nd.accept     = true
+ne.accept     = true
+ng.accept     = true
+nl.accept     = true
+nl-be.accept  = true
+nn.accept     = true
+no.accept     = true
+nr.accept     = true
+nso.accept    = true
+nv.accept     = true
+ny.accept     = true
+oc.accept     = true
+oj.accept     = true
+om.accept     = true
+or.accept     = true
+os.accept     = true
+pa.accept     = true
+pa-in.accept  = true
+pa-pk.accept  = true
+pi.accept     = true
+pl.accept     = true
+ps.accept     = true
+pt.accept     = true
+pt-br.accept  = true
+pt-pt.accept  = true
+qu.accept     = true
+rm.accept     = true
+rn.accept     = true
+ro.accept     = true
+ro-md.accept  = true
+ro-ro.accept  = true
+ru.accept     = true
+ru-md.accept  = true
+rw.accept     = true
+sa.accept     = true
+sat.accept    = true
+sc.accept     = true
+sco.accept    = true
+sd.accept     = true
+sg.accept     = true
+si.accept     = true
+sk.accept     = true
+sl.accept     = true
+sm.accept     = true
+so.accept     = true
+son.accept    = true
+son-ml.accept = true
+sq.accept     = true
+sr.accept     = true
+ss.accept     = true
+st.accept     = true
+su.accept     = true
+sv.accept     = true
+sv-fi.accept  = true
+sv-se.accept  = true
+sw.accept     = true
+szl.accept    = true
+ta.accept     = true
+te.accept     = true
+tg.accept     = true
+th.accept     = true
+ti.accept     = true
+tig.accept    = true
+tk.accept     = true
+tl.accept     = true
+tlh.accept    = true
+tn.accept     = true
+to.accept     = true
+tr.accept     = true
+trs.accept    = true
+ts.accept     = true
+tt.accept     = true
+tw.accept     = true
+ty.accept     = true
+ug.accept     = true
+uk.accept     = true
+ur.accept     = true
+uz.accept     = true
+ve.accept     = true
+vi.accept     = true
+vo.accept     = true
+wa.accept     = true
+wo.accept     = true
+xh.accept     = true
+yi.accept     = true
+yo.accept     = true
+za.accept     = true
+zam.accept    = true
+zh.accept     = true
+zh-cn.accept  = true
+zh-hk.accept  = true
+zh-sg.accept  = true
+zh-tw.accept  = true
+zu.accept     = true
diff --git a/intl/locale/mac/OSPreferences_mac.cpp b/intl/locale/mac/OSPreferences_mac.cpp
new file mode 100644
index 0000000000..f8cd910b40
--- /dev/null
+++ b/intl/locale/mac/OSPreferences_mac.cpp
@@ -0,0 +1,158 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "OSPreferences.h"
+#include "mozilla/intl/LocaleService.h"
+#include <Carbon/Carbon.h>
+
+using namespace mozilla::intl;
+
+static void LocaleChangedNotificationCallback(CFNotificationCenterRef center,
+                                              void* observer, CFStringRef name,
+                                              const void* object,
+                                              CFDictionaryRef userInfo) {
+  if (!::CFEqual(name, kCFLocaleCurrentLocaleDidChangeNotification)) {
+    return;
+  }
+  static_cast<OSPreferences*>(observer)->Refresh();
+}
+
+OSPreferences::OSPreferences() {
+  ::CFNotificationCenterAddObserver(
+      ::CFNotificationCenterGetLocalCenter(), this,
+      LocaleChangedNotificationCallback,
+      kCFLocaleCurrentLocaleDidChangeNotification, 0,
+      CFNotificationSuspensionBehaviorDeliverImmediately);
+}
+
+bool OSPreferences::ReadSystemLocales(nsTArray<nsCString>& aLocaleList) {
+  MOZ_ASSERT(aLocaleList.IsEmpty());
+
+  CFArrayRef langs = ::CFLocaleCopyPreferredLanguages();
+  for (CFIndex i = 0; i < ::CFArrayGetCount(langs); i++) {
+    CFStringRef lang = (CFStringRef)::CFArrayGetValueAtIndex(langs, i);
+
+    AutoTArray<UniChar, 32> buffer;
+    int size = ::CFStringGetLength(lang);
+    buffer.SetLength(size);
+
+    CFRange range = ::CFRangeMake(0, size);
+    ::CFStringGetCharacters(lang, range, buffer.Elements());
+
+    // Convert the locale string to the format that Mozilla expects
+    NS_LossyConvertUTF16toASCII locale(
+        reinterpret_cast<const char16_t*>(buffer.Elements()), buffer.Length());
+
+    if (CanonicalizeLanguageTag(locale)) {
+      aLocaleList.AppendElement(locale);
+    }
+  }
+
+  ::CFRelease(langs);
+
+  return !aLocaleList.IsEmpty();
+}
+
+bool OSPreferences::ReadRegionalPrefsLocales(nsTArray<nsCString>& aLocaleList) {
+  // For now we're just taking System Locales since we don't know of any better
+  // API for regional prefs.
+  return ReadSystemLocales(aLocaleList);
+}
+
+static CFDateFormatterStyle ToCFDateFormatterStyle(
+    OSPreferences::DateTimeFormatStyle aFormatStyle) {
+  switch (aFormatStyle) {
+    case OSPreferences::DateTimeFormatStyle::None:
+      return kCFDateFormatterNoStyle;
+    case OSPreferences::DateTimeFormatStyle::Short:
+      return kCFDateFormatterShortStyle;
+    case OSPreferences::DateTimeFormatStyle::Medium:
+      return kCFDateFormatterMediumStyle;
+    case OSPreferences::DateTimeFormatStyle::Long:
+      return kCFDateFormatterLongStyle;
+    case OSPreferences::DateTimeFormatStyle::Full:
+      return kCFDateFormatterFullStyle;
+    case OSPreferences::DateTimeFormatStyle::Invalid:
+      MOZ_ASSERT_UNREACHABLE("invalid time format");
+      return kCFDateFormatterNoStyle;
+  }
+}
+
+// Given an 8-bit Gecko string, create a corresponding CFLocale;
+// if aLocale is empty, returns a copy of the system's current locale.
+// May return null on failure.
+// Follows Core Foundation's Create rule, so the caller is responsible to
+// release the returned reference.
+static CFLocaleRef CreateCFLocaleFor(const nsACString& aLocale) {
+  nsAutoCString reqLocale;
+  nsAutoCString systemLocale;
+
+  OSPreferences::GetInstance()->GetSystemLocale(systemLocale);
+
+  if (aLocale.IsEmpty()) {
+    LocaleService::GetInstance()->GetAppLocaleAsBCP47(reqLocale);
+  } else {
+    reqLocale.Assign(aLocale);
+  }
+
+  bool match = LocaleService::LanguagesMatch(reqLocale, systemLocale);
+  if (match) {
+    return ::CFLocaleCopyCurrent();
+  }
+
+  CFStringRef identifier = CFStringCreateWithBytesNoCopy(
+      kCFAllocatorDefault, (const uint8_t*)reqLocale.BeginReading(),
+      reqLocale.Length(), kCFStringEncodingASCII, false, kCFAllocatorNull);
+  if (!identifier) {
+    return nullptr;
+  }
+  CFLocaleRef locale = CFLocaleCreate(kCFAllocatorDefault, identifier);
+  CFRelease(identifier);
+  return locale;
+}
+
+/**
+ * Cocoa API maps nicely to our four styles of date/time.
+ *
+ * The only caveat is that Cocoa takes regional preferences modifications
+ * into account only when we pass an empty string as a locale.
+ *
+ * In all other cases it will return the default pattern for a given locale.
+ */
+bool OSPreferences::ReadDateTimePattern(DateTimeFormatStyle aDateStyle,
+                                        DateTimeFormatStyle aTimeStyle,
+                                        const nsACString& aLocale,
+                                        nsACString& aRetVal) {
+  CFLocaleRef locale = CreateCFLocaleFor(aLocale);
+  if (!locale) {
+    return false;
+  }
+
+  CFDateFormatterRef formatter = CFDateFormatterCreate(
+      kCFAllocatorDefault, locale, ToCFDateFormatterStyle(aDateStyle),
+      ToCFDateFormatterStyle(aTimeStyle));
+  if (!formatter) {
+    return false;
+  }
+  CFStringRef format = CFDateFormatterGetFormat(formatter);
+  CFRelease(locale);
+
+  CFRange range = CFRangeMake(0, CFStringGetLength(format));
+  nsAutoString str;
+  str.SetLength(range.length);
+  CFStringGetCharacters(format, range,
+                        reinterpret_cast<UniChar*>(str.BeginWriting()));
+  CFRelease(formatter);
+
+  aRetVal = NS_ConvertUTF16toUTF8(str);
+  return true;
+}
+
+void OSPreferences::RemoveObservers() {
+  ::CFNotificationCenterRemoveObserver(
+      ::CFNotificationCenterGetLocalCenter(), this,
+      kCTFontManagerRegisteredFontsChangedNotification, 0);
+}
diff --git a/intl/locale/mac/moz.build b/intl/locale/mac/moz.build
new file mode 100644
index 0000000000..68eb6d8d35
--- /dev/null
+++ b/intl/locale/mac/moz.build
@@ -0,0 +1,12 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+UNIFIED_SOURCES += ["OSPreferences_mac.cpp"]
+
+FINAL_LIBRARY = "xul"
+LOCAL_INCLUDES += [
+    "..",
+]
diff --git a/intl/locale/moz.build b/intl/locale/moz.build
new file mode 100644
index 0000000000..8099d1a695
--- /dev/null
+++ b/intl/locale/moz.build
@@ -0,0 +1,97 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+XPCSHELL_TESTS_MANIFESTS += ["tests/unit/xpcshell.toml"]
+
+TESTING_JS_MODULES += [
+    "tests/LangPackMatcherTestUtils.sys.mjs",
+]
+
+toolkit = CONFIG["MOZ_WIDGET_TOOLKIT"]
+
+if toolkit == "windows":
+    DIRS += ["windows"]
+elif toolkit == "cocoa":
+    DIRS += ["mac"]
+elif toolkit == "gtk":
+    DIRS += ["gtk"]
+elif toolkit == "android":
+    DIRS += ["android"]
+
+XPIDL_SOURCES += [
+    "mozILocaleService.idl",
+    "mozIOSPreferences.idl",
+]
+
+XPIDL_MODULE = "locale"
+
+EXPORTS += [
+    "nsLanguageAtomService.h",
+    "nsUConvPropertySearch.h",
+]
+
+EXPORTS.mozilla.intl += [
+    "AppDateTimeFormat.h",
+    "LocaleService.h",
+    "MozLocaleBindings.h",
+    "OSPreferences.h",
+    "Quotes.h",
+]
+
+UNIFIED_SOURCES += [
+    "AppDateTimeFormat.cpp",
+    "LocaleService.cpp",
+    "nsLanguageAtomService.cpp",
+    "nsUConvPropertySearch.cpp",
+    "OSPreferences.cpp",
+    "Quotes.cpp",
+]
+
+EXTRA_JS_MODULES += [
+    "LangPackMatcher.sys.mjs",
+]
+
+XPCOM_MANIFESTS += [
+    "components.conf",
+]
+
+FINAL_LIBRARY = "xul"
+
+LOCAL_INCLUDES += [
+    "/intl/uconv",
+]
+
+RESOURCE_FILES += [
+    "language.properties",
+]
+
+prefixes = ("encodingsgroups",)
+
+for prefix in prefixes:
+    input_file = prefix + ".properties"
+    header = prefix + ".properties.h"
+    GeneratedFile(header, script="props2arrays.py", inputs=[input_file])
+
+if CONFIG["ENABLE_TESTS"]:
+    DIRS += ["tests/gtest"]
+
+if CONFIG["COMPILE_ENVIRONMENT"]:
+    CbindgenHeader(
+        "fluent_langneg_ffi_generated.h",
+        inputs=["/intl/locale/rust/fluent-langneg-ffi"],
+    )
+    CbindgenHeader(
+        "oxilangtag_ffi_generated.h", inputs=["/intl/locale/rust/oxilangtag-ffi"]
+    )
+    CbindgenHeader(
+        "unic_langid_ffi_generated.h", inputs=["/intl/locale/rust/unic-langid-ffi"]
+    )
+
+    EXPORTS.mozilla.intl += [
+        "!fluent_langneg_ffi_generated.h",
+        "!oxilangtag_ffi_generated.h",
+        "!unic_langid_ffi_generated.h",
+    ]
diff --git a/intl/locale/mozILocaleService.idl b/intl/locale/mozILocaleService.idl
new file mode 100644
index 0000000000..71227a64d3
--- /dev/null
+++ b/intl/locale/mozILocaleService.idl
@@ -0,0 +1,185 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+%{C++
+// Define Contractid and CID
+#define MOZ_LOCALESERVICE_CID \
+  { 0x92735ff4, 0x6384, 0x4ad6, { 0x85, 0x08, 0x75, 0x70, 0x10, 0xe1, 0x49, 0xee } }
+
+#define MOZ_LOCALESERVICE_CONTRACTID "@mozilla.org/intl/localeservice;1"
+%}
+
+[scriptable, uuid(C27F8983-B48B-4D1A-92D7-FEB8106F212D)]
+interface mozILocaleService : nsISupports
+{
+  /**
+   * List of language negotiation strategies to use.
+   * For an example list of requested and available locales:
+   *
+   *   Requested: ['es-MX', 'fr-FR']
+   *   Available: ['fr', 'fr-CA', 'es', 'es-MX', 'it']
+   *   DefaultLocale: ['en-US']
+   *
+   * each of those strategies will build a different result:
+   *
+   *
+   * filtering (default) -
+   *   Matches as many of the available locales as possible.
+   *
+   *   Result:
+   *     Supported: ['es-MX', 'es', 'fr', 'fr-CA', 'en-US']
+   *
+   * matching -
+   *   Matches the best match from the available locales for every requested
+   *   locale.
+   *
+   *   Result:
+   *     Supported: ['es-MX', 'fr', 'en-US']
+   *
+   * lookup -
+   *   Matches a single best locale. This strategy always returns a list
+   *   of the length 1 and requires a defaultLocale to be set.
+   *
+   *   Result:
+   *     Supported: ['es-MX']
+   */
+  const long langNegStrategyFiltering = 0;
+  const long langNegStrategyMatching  = 1;
+  const long langNegStrategyLookup    = 2;
+
+  /**
+   * Default locale of the browser. The locale we are guaranteed to have
+   * resources for that should be used as a last resort fallack in cases
+   * where requested locales do not match available locales.
+   */
+  readonly attribute ACString defaultLocale;
+
+  /**
+   * Last fallback is the final fallback locale we're going to attempt if all
+   * else fails in any language negotiation or locale resource retrieval situations.
+   *
+   * At the moment it returns `en-US`.
+   */
+  readonly attribute ACString lastFallbackLocale;
+
+  /**
+   * Returns a list of locales that the application should be localized to.
+   *
+   * The result is a ordered list of valid locale IDs and it should be
+   * used for all APIs that accept list of locales, like ECMA402 and L10n APIs.
+   *
+   * This API always returns at least one locale.
+   *
+   * When retrieving the locales for language negotiation and matching
+   * to language resources, use the language tag form.
+   * When retrieving the locales for Intl API or ICU locale settings,
+   * use the BCP47 form.
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   */
+  readonly attribute Array<ACString> appLocalesAsLangTags;
+  readonly attribute Array<ACString> appLocalesAsBCP47;
+
+  /**
+   * Returns a list of locales to use for any regional specific operations
+   * like date formatting, calendars, unit formatting etc.
+   *
+   * The result is a ordered list of valid locale IDs and it should be
+   * used for all APIs that accept list of locales, like ECMA402 and L10n APIs.
+   *
+   * This API always returns at least one locale.
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   */
+  readonly attribute Array<ACString> regionalPrefsLocales;
+
+  readonly attribute Array<ACString> webExposedLocales;
+
+  /**
+   * Negotiates the best locales out of a ordered list of requested locales and
+   * a list of available locales.
+   *
+   * Internally it uses the following naming scheme:
+   *
+   *  Requested - locales requested by the user
+   *  Available - locales for which the data is available
+   *  Supported - locales negotiated by the algorithm
+   *
+   * Additionally, if defaultLocale is provided, it adds it to the end of the
+   * result list as a "last resort" locale.
+   *
+   * Strategy is one of the three strategies described at the top of this file.
+   *
+   * The result list is canonicalized and ordered according to the order
+   * of the requested locales.
+   *
+   * (See LocaleService.h for a more C++-friendly version of this.)
+   */
+  Array<ACString> negotiateLanguages(in Array<AUTF8String> aRequested,
+                                     in Array<AUTF8String> aAvailable,
+                                     [optional] in ACString aDefaultLocale,
+                                     [optional] in long langNegStrategy);
+
+  /**
+   * Returns the best locale that the application should be localized to.
+   *
+   * The result is a valid locale ID and it should be
+   * used for all APIs that do not handle language negotiation.
+   *
+   * When retrieving the locales for language negotiation and matching
+   * to language resources, use the language tag form.
+   * When retrieving the locales for Intl API or ICU locale settings,
+   * use the BCP47 form.
+   *
+   * Where possible, appLocales* should be preferred over this API and
+   * all callsites should handle some form of "best effort" language
+   * negotiation to respect user preferences in case the use case does
+   * not have data for the first locale in the list.
+   *
+   * Example: "zh-Hans-HK"
+   */
+  readonly attribute ACString appLocaleAsLangTag;
+  readonly attribute ACString appLocaleAsBCP47;
+
+  /**
+   * Returns a list of locales that the user requested the app to be
+   * localized to.
+   *
+   * The result is an ordered list of locale IDs which should be
+   * used as a requestedLocales input list for language negotiation.
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   */
+  attribute Array<ACString> requestedLocales;
+
+  /**
+   * Returns the top-requested locale from the user, or an empty string if none is set.
+   */
+  readonly attribute ACString requestedLocale;
+
+  /**
+   * Returns a list of locales that the app can be localized to.
+   *
+   * The result is an unordered list of locale IDs which should be
+   * used as a availableLocales input list for language negotiation.
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   */
+  attribute Array<ACString> availableLocales;
+
+  /**
+   * Returns whether the current app locale is RTL.
+   */
+  readonly attribute boolean isAppLocaleRTL;
+
+  /**
+   * Returns a list of locales packaged into the app bundle.
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   */
+  readonly attribute Array<ACString> packagedLocales;
+};
diff --git a/intl/locale/mozIOSPreferences.idl b/intl/locale/mozIOSPreferences.idl
new file mode 100644
index 0000000000..f3e1c12440
--- /dev/null
+++ b/intl/locale/mozIOSPreferences.idl
@@ -0,0 +1,95 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+%{C++
+// Define Contractid and CID
+#define MOZ_OSPREFERENCES_CID \
+  { 0x65944815, 0xe9ae, 0x48bd, { 0xa2, 0xbf, 0xf1, 0x10, 0x87, 0x20, 0x95, 0x0c } }
+
+#define MOZ_OSPREFERENCES_CONTRACTID "@mozilla.org/intl/ospreferences;1"
+%}
+
+[scriptable, uuid(65944815-e9ae-48bd-a2bf-f1108720950c)]
+interface mozIOSPreferences : nsISupports
+{
+  const long dateTimeFormatStyleNone     = 0;
+  const long dateTimeFormatStyleShort    = 1;
+  const long dateTimeFormatStyleMedium   = 2;
+  const long dateTimeFormatStyleLong     = 3;
+  const long dateTimeFormatStyleFull     = 4;
+
+  /**
+   * Returns a list of locales used by the host environment for UI
+   * localization.
+   *
+   * The result is a sorted list and we expect that the OS attempts to
+   * use the top locale from the list for which it has data.
+   *
+   * Each element of the list is a valid locale ID that can be passed to ICU
+   * and ECMA402 Intl APIs,
+   * At the same time each element is a valid BCP47 language tag that can be
+   * used for language negotiation.
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   */
+  readonly attribute Array<ACString> systemLocales;
+
+  /**
+   * Returns a list of locales used by host environment for regional
+   * preferences internationalization.
+   *
+   * The result is a sorted list and we expect that the OS attempts to
+   * use the top locale from the list for which it has data.
+   *
+   * Each element of the list is a valid locale ID that can be passed to ICU
+   * and ECMA402 Intl APIs,
+   *
+   * Example: ["en-US", "de", "pl", "sr-Cyrl", "zh-Hans-HK"]
+   */
+  readonly attribute Array<ACString> regionalPrefsLocales;
+
+  /**
+   * Returns the best locale that the host environment is localized to.
+   *
+   * The result is a valid locale ID and it should be
+   * used for all APIs that do not handle language negotiation.
+   *
+   * In any scenario involving language negotiation, systemLocales should
+   * be preferred over the single value.
+   *
+   * Example: "zh-Hans-HK"
+   */
+  readonly attribute ACString systemLocale;
+
+  /**
+   * Returns the best possible date/time pattern for the host environment
+   * taking into account date/time regional settings user defined in the OS
+   * preferences.
+   *
+   * Notice, that depending on the OS it may take into account those settings
+   * for all locales, or only if the locale matches the OS locale.
+   *
+   * It takes two integer arguments that must be valid `dateTimeFormatStyle*`
+   * values (see constants defined above), and a string representing a
+   * BCP47 locale.
+   *
+   * It returns a string with a LDML date/time pattern.
+   *
+   * If no pattern can be retrieved from the host environment, it will
+   * lookup the best available pattern from ICU.
+   *
+   * Notice, this is a pretty unique method in this API in that it does
+   * more than look up into host environment.
+   * The reason for that is that constructing the right date/time pattern
+   * requires a lot of OS-specific logic and it ends up being easier to just
+   * handle all scenarios, including with cases where we fail to retrieve
+   * anything from the OS, here.
+   */
+  AUTF8String getDateTimePattern(in long timeFormatStyle,
+                                 in long dateFormatStyle,
+                                 [optional] in ACString locale);
+};
diff --git a/intl/locale/nsLanguageAtomService.cpp b/intl/locale/nsLanguageAtomService.cpp
new file mode 100644
index 0000000000..6c57fb8743
--- /dev/null
+++ b/intl/locale/nsLanguageAtomService.cpp
@@ -0,0 +1,256 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsLanguageAtomService.h"
+#include "nsUConvPropertySearch.h"
+#include "nsUnicharUtils.h"
+#include "nsAtom.h"
+#include "nsGkAtoms.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/Encoding.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/intl/OSPreferences.h"
+#include "mozilla/ServoBindings.h"
+#include "mozilla/ServoUtils.h"
+#include "mozilla/StaticPtr.h"
+
+using namespace mozilla;
+using mozilla::intl::OSPreferences;
+
+static constexpr nsUConvProp encodingsGroups[] = {
+#include "encodingsgroups.properties.h"
+};
+
+// List of mozilla internal x-* tags that map to themselves (see bug 256257)
+static constexpr nsStaticAtom* kLangGroups[] = {
+    // This list must be sorted!
+    nsGkAtoms::x_armn,  nsGkAtoms::x_cyrillic, nsGkAtoms::x_devanagari,
+    nsGkAtoms::x_geor,  nsGkAtoms::x_math,     nsGkAtoms::x_tamil,
+    nsGkAtoms::Unicode, nsGkAtoms::x_western
+    // These self-mappings are not necessary unless somebody use them to specify
+    // lang in (X)HTML/XML documents, which they shouldn't. (see bug 256257)
+    // x-beng=x-beng
+    // x-cans=x-cans
+    // x-ethi=x-ethi
+    // x-guru=x-guru
+    // x-gujr=x-gujr
+    // x-khmr=x-khmr
+    // x-mlym=x-mlym
+};
+
+// Map ISO 15924 script codes from BCP47 lang tag to mozilla's langGroups.
+static constexpr struct {
+  const char* mTag;
+  nsStaticAtom* mAtom;
+} kScriptLangGroup[] = {
+    // This list must be sorted by script code!
+    {"Arab", nsGkAtoms::ar},
+    {"Armn", nsGkAtoms::x_armn},
+    {"Beng", nsGkAtoms::x_beng},
+    {"Cans", nsGkAtoms::x_cans},
+    {"Cyrl", nsGkAtoms::x_cyrillic},
+    {"Deva", nsGkAtoms::x_devanagari},
+    {"Ethi", nsGkAtoms::x_ethi},
+    {"Geok", nsGkAtoms::x_geor},
+    {"Geor", nsGkAtoms::x_geor},
+    {"Grek", nsGkAtoms::el},
+    {"Gujr", nsGkAtoms::x_gujr},
+    {"Guru", nsGkAtoms::x_guru},
+    {"Hang", nsGkAtoms::ko},
+    // Hani is not mapped to a specific langGroup, we prefer to look at the
+    // primary language subtag in this case
+    {"Hans", nsGkAtoms::Chinese},
+    // Hant is special-cased in code
+    // Hant=zh-HK
+    // Hant=zh-TW
+    {"Hebr", nsGkAtoms::he},
+    {"Hira", nsGkAtoms::Japanese},
+    {"Jpan", nsGkAtoms::Japanese},
+    {"Kana", nsGkAtoms::Japanese},
+    {"Khmr", nsGkAtoms::x_khmr},
+    {"Knda", nsGkAtoms::x_knda},
+    {"Kore", nsGkAtoms::ko},
+    {"Latn", nsGkAtoms::x_western},
+    {"Mlym", nsGkAtoms::x_mlym},
+    {"Orya", nsGkAtoms::x_orya},
+    {"Sinh", nsGkAtoms::x_sinh},
+    {"Taml", nsGkAtoms::x_tamil},
+    {"Telu", nsGkAtoms::x_telu},
+    {"Thai", nsGkAtoms::th},
+    {"Tibt", nsGkAtoms::x_tibt}};
+
+static StaticAutoPtr<nsLanguageAtomService> gLangAtomService;
+
+// static
+nsLanguageAtomService* nsLanguageAtomService::GetService() {
+  if (!gLangAtomService) {
+    gLangAtomService = new nsLanguageAtomService();
+  }
+  return gLangAtomService.get();
+}
+
+// static
+void nsLanguageAtomService::Shutdown() { gLangAtomService = nullptr; }
+
+nsStaticAtom* nsLanguageAtomService::LookupLanguage(
+    const nsACString& aLanguage) {
+  nsAutoCString lowered(aLanguage);
+  ToLowerCase(lowered);
+
+  RefPtr<nsAtom> lang = NS_Atomize(lowered);
+  return GetLanguageGroup(lang);
+}
+
+already_AddRefed<nsAtom> nsLanguageAtomService::LookupCharSet(
+    NotNull<const Encoding*> aEncoding) {
+  nsAutoCString charset;
+  aEncoding->Name(charset);
+  nsAutoCString group;
+  if (NS_FAILED(nsUConvPropertySearch::SearchPropertyValue(
+          encodingsGroups, ArrayLength(encodingsGroups), charset, group))) {
+    return RefPtr<nsAtom>(nsGkAtoms::Unicode).forget();
+  }
+  return NS_Atomize(group);
+}
+
+nsAtom* nsLanguageAtomService::GetLocaleLanguage() {
+  do {
+    if (!mLocaleLanguage) {
+      AutoTArray<nsCString, 10> regionalPrefsLocales;
+      if (NS_SUCCEEDED(OSPreferences::GetInstance()->GetRegionalPrefsLocales(
+              regionalPrefsLocales))) {
+        // use lowercase for all language atoms
+        ToLowerCase(regionalPrefsLocales[0]);
+        mLocaleLanguage = NS_Atomize(regionalPrefsLocales[0]);
+      } else {
+        nsAutoCString locale;
+        OSPreferences::GetInstance()->GetSystemLocale(locale);
+
+        ToLowerCase(locale);  // use lowercase for all language atoms
+        mLocaleLanguage = NS_Atomize(locale);
+      }
+    }
+  } while (0);
+
+  return mLocaleLanguage;
+}
+
+nsStaticAtom* nsLanguageAtomService::GetLanguageGroup(nsAtom* aLanguage,
+                                                      bool* aNeedsToCache) {
+  if (aNeedsToCache) {
+    if (nsStaticAtom* atom = mLangToGroup.Get(aLanguage)) {
+      return atom;
+    }
+    *aNeedsToCache = true;
+    return nullptr;
+  }
+
+  return mLangToGroup.LookupOrInsertWith(aLanguage, [&] {
+    AssertIsMainThreadOrServoFontMetricsLocked();
+    return GetUncachedLanguageGroup(aLanguage);
+  });
+}
+
+nsStaticAtom* nsLanguageAtomService::GetUncachedLanguageGroup(
+    nsAtom* aLanguage) const {
+  nsAutoCString langStr;
+  aLanguage->ToUTF8String(langStr);
+  ToLowerCase(langStr);
+
+  if (langStr[0] == 'x' && langStr[1] == '-') {
+    // Internal x-* langGroup codes map to themselves (see bug 256257)
+    for (nsStaticAtom* langGroup : kLangGroups) {
+      if (langGroup == aLanguage) {
+        return langGroup;
+      }
+      if (aLanguage->IsAsciiLowercase()) {
+        continue;
+      }
+      // Do the slow ascii-case-insensitive comparison just if needed.
+      nsDependentAtomString string(langGroup);
+      if (string.EqualsASCII(langStr.get(), langStr.Length())) {
+        return langGroup;
+      }
+    }
+  } else {
+    // If the lang code can be parsed as BCP47, look up its (likely) script.
+
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1618034:
+    // First strip any private subtags that would cause Locale to reject the
+    // tag as non-wellformed.
+    nsACString::const_iterator start, end;
+    langStr.BeginReading(start);
+    langStr.EndReading(end);
+    if (FindInReadable("-x-"_ns, start, end)) {
+      // The substring we want ends at the beginning of the "-x-" subtag.
+      langStr.Truncate(start.get() - langStr.BeginReading());
+    }
+
+    intl::Locale loc;
+    auto result = intl::LocaleParser::TryParse(langStr, loc);
+    if (!result.isOk()) {
+      // Did the author (wrongly) use '_' instead of '-' to separate subtags?
+      // If so, fix it up and re-try parsing.
+      if (langStr.Contains('_')) {
+        langStr.ReplaceChar('_', '-');
+
+        // Throw away the partially parsed locale and re-start parsing.
+        loc = {};
+        result = intl::LocaleParser::TryParse(langStr, loc);
+      }
+    }
+    if (result.isOk() && loc.Canonicalize().isOk()) {
+      // Fill in script subtag if not present.
+      if (loc.Script().Missing()) {
+        if (loc.AddLikelySubtags().isErr()) {
+          // Fall back to x-unicode if no match was found
+          return nsGkAtoms::Unicode;
+        }
+      }
+      // Traditional Chinese has separate prefs for Hong Kong / Taiwan;
+      // check the region subtag.
+      if (loc.Script().EqualTo("Hant")) {
+        if (loc.Region().EqualTo("HK")) {
+          return nsGkAtoms::HongKongChinese;
+        }
+        return nsGkAtoms::Taiwanese;
+      }
+      // Search list of known script subtags that map to langGroup codes.
+      size_t foundIndex;
+      Span<const char> scriptAsSpan = loc.Script().Span();
+      nsDependentCSubstring script(scriptAsSpan.data(), scriptAsSpan.size());
+      if (BinarySearchIf(
+              kScriptLangGroup, 0, ArrayLength(kScriptLangGroup),
+              [script](const auto& entry) -> int {
+                return Compare(script, nsDependentCString(entry.mTag));
+              },
+              &foundIndex)) {
+        return kScriptLangGroup[foundIndex].mAtom;
+      }
+      // Script subtag was not recognized (includes "Hani"); check the language
+      // subtag for CJK possibilities so that we'll prefer the appropriate font
+      // rather than falling back to the browser's hardcoded preference.
+      if (loc.Language().EqualTo("zh")) {
+        if (loc.Region().EqualTo("HK")) {
+          return nsGkAtoms::HongKongChinese;
+        }
+        if (loc.Region().EqualTo("TW")) {
+          return nsGkAtoms::Taiwanese;
+        }
+        return nsGkAtoms::Chinese;
+      }
+      if (loc.Language().EqualTo("ja")) {
+        return nsGkAtoms::Japanese;
+      }
+      if (loc.Language().EqualTo("ko")) {
+        return nsGkAtoms::ko;
+      }
+    }
+  }
+
+  // Fall back to x-unicode if no match was found
+  return nsGkAtoms::Unicode;
+}
diff --git a/intl/locale/nsLanguageAtomService.h b/intl/locale/nsLanguageAtomService.h
new file mode 100644
index 0000000000..521ead4e87
--- /dev/null
+++ b/intl/locale/nsLanguageAtomService.h
@@ -0,0 +1,61 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * The nsILanguageAtomService provides a mapping from languages or charsets
+ * to language groups, and access to the system locale language.
+ */
+
+#ifndef nsLanguageAtomService_h_
+#define nsLanguageAtomService_h_
+
+#include "mozilla/NotNull.h"
+#include "nsCOMPtr.h"
+#include "nsAtomHashKeys.h"
+#include "nsTHashMap.h"
+
+namespace mozilla {
+class Encoding;
+}
+
+class nsLanguageAtomService final {
+  using Encoding = mozilla::Encoding;
+  template <typename T>
+  using NotNull = mozilla::NotNull<T>;
+
+ public:
+  static nsLanguageAtomService* GetService();
+
+  static void Shutdown();
+
+  nsStaticAtom* LookupLanguage(const nsACString& aLanguage);
+  already_AddRefed<nsAtom> LookupCharSet(NotNull<const Encoding*> aCharSet);
+  nsAtom* GetLocaleLanguage();
+
+  // Returns the language group that the specified language is a part of.
+  //
+  // aNeedsToCache is used for two things.  If null, it indicates that
+  // the nsLanguageAtomService is safe to cache the result of the
+  // language group lookup, either because we're on the main thread,
+  // or because we're on a style worker thread but the font lock has
+  // been acquired.  If non-null, it indicates that it's not safe to
+  // cache the result of the language group lookup (because we're on
+  // a style worker thread without the lock acquired).  In this case,
+  // GetLanguageGroup will store true in *aNeedsToCache true if we
+  // would have cached the result of a new lookup, and false if we
+  // were able to use an existing cached result.  Thus, callers that
+  // get a true *aNeedsToCache outparam value should make an effort
+  // to re-call GetLanguageGroup when it is safe to cache, to avoid
+  // recomputing the language group again later.
+  nsStaticAtom* GetLanguageGroup(nsAtom* aLanguage,
+                                 bool* aNeedsToCache = nullptr);
+  nsStaticAtom* GetUncachedLanguageGroup(nsAtom* aLanguage) const;
+
+ private:
+  nsTHashMap<RefPtr<nsAtom>, nsStaticAtom*> mLangToGroup;
+  RefPtr<nsAtom> mLocaleLanguage;
+};
+
+#endif
diff --git a/intl/locale/nsUConvPropertySearch.cpp b/intl/locale/nsUConvPropertySearch.cpp
new file mode 100644
index 0000000000..114b85e0fe
--- /dev/null
+++ b/intl/locale/nsUConvPropertySearch.cpp
@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsUConvPropertySearch.h"
+#include "nsCRT.h"
+#include "nsString.h"
+#include "mozilla/BinarySearch.h"
+
+namespace {
+
+struct PropertyComparator {
+  const nsCString& mKey;
+  explicit PropertyComparator(const nsCString& aKey) : mKey(aKey) {}
+  int operator()(const nsUConvProp& aProperty) const {
+    return Compare(mKey, nsDependentCString(aProperty.mKey));
+  }
+};
+
+}  // namespace
+
+// static
+nsresult nsUConvPropertySearch::SearchPropertyValue(
+    const nsUConvProp aProperties[], int32_t aNumberOfProperties,
+    const nsACString& aKey, nsACString& aValue) {
+  using mozilla::BinarySearchIf;
+
+  const nsCString& flat = PromiseFlatCString(aKey);
+  size_t index;
+  if (BinarySearchIf(aProperties, 0, aNumberOfProperties,
+                     PropertyComparator(flat), &index)) {
+    nsDependentCString val(aProperties[index].mValue,
+                           aProperties[index].mValueLength);
+    aValue.Assign(val);
+    return NS_OK;
+  }
+
+  aValue.Truncate();
+  return NS_ERROR_FAILURE;
+}
diff --git a/intl/locale/nsUConvPropertySearch.h b/intl/locale/nsUConvPropertySearch.h
new file mode 100644
index 0000000000..4914deb5ec
--- /dev/null
+++ b/intl/locale/nsUConvPropertySearch.h
@@ -0,0 +1,35 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsUConvPropertySearch_h_
+#define nsUConvPropertySearch_h_
+
+#include "nsStringFwd.h"
+
+struct nsUConvProp {
+  const char* const mKey;
+  const char* const mValue;
+  const uint32_t mValueLength;
+};
+
+class nsUConvPropertySearch {
+ public:
+  /**
+   * Looks up a property by value.
+   *
+   * @param aProperties
+   *   the static property array
+   * @param aKey
+   *   the key to look up
+   * @param aValue
+   *   the return value (empty string if not found)
+   * @return NS_OK if found or NS_ERROR_FAILURE if not found
+   */
+  static nsresult SearchPropertyValue(const nsUConvProp aProperties[],
+                                      int32_t aNumberOfProperties,
+                                      const nsACString& aKey,
+                                      nsACString& aValue);
+};
+
+#endif /* nsUConvPropertySearch_h_ */
diff --git a/intl/locale/props2arrays.py b/intl/locale/props2arrays.py
new file mode 100644
index 0000000000..0572cb1c5f
--- /dev/null
+++ b/intl/locale/props2arrays.py
@@ -0,0 +1,26 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+
+def main(header, propFile):
+    mappings = {}
+
+    with open(propFile, "r") as f:
+        for line in f:
+            line = line.strip()
+            if not line.startswith("#"):
+                parts = line.split("=", 1)
+                if len(parts) == 2 and len(parts[0]) > 0:
+                    mappings[parts[0].strip()] = parts[1].strip()
+
+    keys = mappings.keys()
+
+    header.write("// This is a generated file. Please do not edit.\n")
+    header.write("// Please edit the corresponding .properties file instead.\n")
+
+    entries = [
+        '{ "%s", "%s", %d }' % (key, mappings[key], len(mappings[key]))
+        for key in sorted(keys)
+    ]
+    header.write(",\n".join(entries) + "\n")
diff --git a/intl/locale/rust/fluent-langneg-ffi/Cargo.toml b/intl/locale/rust/fluent-langneg-ffi/Cargo.toml
new file mode 100644
index 0000000000..88d6bad4d4
--- /dev/null
+++ b/intl/locale/rust/fluent-langneg-ffi/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "fluent-langneg-ffi"
+version = "0.1.0"
+license = "MPL-2.0"
+authors = ["Zibi Braniecki <zibi@braniecki.net>"]
+edition = "2018"
+
+[dependencies]
+nsstring = { path = "../../../../xpcom/rust/nsstring" }
+thin-vec = { version = "0.2.1", features = ["gecko-ffi"] }
+fluent-langneg = { version = "0.13", features = ["cldr"] }
+unic-langid = "0.9"
+unic-langid-ffi = { path = "../unic-langid-ffi" }
diff --git a/intl/locale/rust/fluent-langneg-ffi/cbindgen.toml b/intl/locale/rust/fluent-langneg-ffi/cbindgen.toml
new file mode 100644
index 0000000000..98ec15d389
--- /dev/null
+++ b/intl/locale/rust/fluent-langneg-ffi/cbindgen.toml
@@ -0,0 +1,17 @@
+header = """/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */"""
+autogen_warning = """/* DO NOT MODIFY THIS MANUALLY! This file was generated using cbindgen. See RunCbindgen.py */
+#ifndef mozilla_intl_locale_MozLocaleBindings_h
+#error "Don't include this file directly, instead include MozLocaleBindings.h"
+#endif
+"""
+include_version = true
+braces = "SameLine"
+line_length = 100
+tab_width = 2
+language = "C++"
+namespaces = ["mozilla", "intl", "ffi"]
+
+[export.rename]
+"ThinVec" = "nsTArray"
diff --git a/intl/locale/rust/fluent-langneg-ffi/src/lib.rs b/intl/locale/rust/fluent-langneg-ffi/src/lib.rs
new file mode 100644
index 0000000000..591e9ef861
--- /dev/null
+++ b/intl/locale/rust/fluent-langneg-ffi/src/lib.rs
@@ -0,0 +1,79 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use fluent_langneg::negotiate::NegotiationStrategy as LangNegNegotiationStrategy;
+use fluent_langneg::negotiate_languages;
+use nsstring::nsACString;
+use nsstring::nsCString;
+use thin_vec::ThinVec;
+use unic_langid::{LanguageIdentifier, LanguageIdentifierError};
+use unic_langid_ffi::new_langid_for_mozilla;
+
+/// We want to return the exact strings that were passed to us out of the
+/// available and default pool. Since for the negotiation we canonicalize them
+/// in `LanguageIdentifier`, this struct will preserve the original, non-canonicalized
+/// string, and then use it to populate return array.
+#[derive(Debug, PartialEq)]
+struct LangIdString<'l> {
+    pub source: &'l nsCString,
+    pub langid: LanguageIdentifier,
+}
+
+impl<'l> LangIdString<'l> {
+    pub fn try_new(s: &'l nsCString) -> Result<Self, LanguageIdentifierError> {
+        new_langid_for_mozilla(s).map(|l| LangIdString {
+            source: s,
+            langid: l,
+        })
+    }
+}
+
+impl<'l> AsRef<LanguageIdentifier> for LangIdString<'l> {
+    fn as_ref(&self) -> &LanguageIdentifier {
+        &self.langid
+    }
+}
+
+#[repr(C)]
+pub enum NegotiationStrategy {
+    Filtering,
+    Matching,
+    Lookup,
+}
+
+fn get_strategy(input: NegotiationStrategy) -> LangNegNegotiationStrategy {
+    match input {
+        NegotiationStrategy::Filtering => LangNegNegotiationStrategy::Filtering,
+        NegotiationStrategy::Matching => LangNegNegotiationStrategy::Matching,
+        NegotiationStrategy::Lookup => LangNegNegotiationStrategy::Lookup,
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn fluent_langneg_negotiate_languages(
+    requested: &ThinVec<nsCString>,
+    available: &ThinVec<nsCString>,
+    default: &nsACString,
+    strategy: NegotiationStrategy,
+    result: &mut ThinVec<nsCString>,
+) {
+    let requested = requested
+        .iter()
+        .filter_map(|s| new_langid_for_mozilla(s).ok())
+        .collect::<Vec<_>>();
+
+    let available = available
+        .iter()
+        .filter_map(|s| LangIdString::try_new(s).ok())
+        .collect::<Vec<_>>();
+
+    let d: nsCString = default.into();
+    let default = LangIdString::try_new(&d).ok();
+
+    let strategy = get_strategy(strategy);
+
+    for l in negotiate_languages(&requested, &available, default.as_ref(), strategy) {
+        result.push(l.source.clone());
+    }
+}
diff --git a/intl/locale/rust/oxilangtag-ffi/Cargo.toml b/intl/locale/rust/oxilangtag-ffi/Cargo.toml
new file mode 100644
index 0000000000..ee3b1cf5c8
--- /dev/null
+++ b/intl/locale/rust/oxilangtag-ffi/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "oxilangtag-ffi"
+version = "0.1.0"
+license = "MPL-2.0"
+authors = ["Jonathan Kew <jkew@mozilla.com>"]
+edition = "2021"
+
+[dependencies]
+nsstring = { path = "../../../../xpcom/rust/nsstring" }
+oxilangtag = "0.1.3"
diff --git a/intl/locale/rust/oxilangtag-ffi/cbindgen.toml b/intl/locale/rust/oxilangtag-ffi/cbindgen.toml
new file mode 100644
index 0000000000..21d703000b
--- /dev/null
+++ b/intl/locale/rust/oxilangtag-ffi/cbindgen.toml
@@ -0,0 +1,15 @@
+header = """/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */"""
+autogen_warning = """/* DO NOT MODIFY THIS MANUALLY! This file was generated using cbindgen. See RunCbindgen.py */
+"""
+include_version = true
+braces = "SameLine"
+line_length = 100
+tab_width = 2
+language = "C++"
+namespaces = ["mozilla", "intl", "ffi"]
+
+[parse]
+parse_deps = true
+include = ["oxilangtag"]
diff --git a/intl/locale/rust/oxilangtag-ffi/src/lib.rs b/intl/locale/rust/oxilangtag-ffi/src/lib.rs
new file mode 100644
index 0000000000..5a30e9b77f
--- /dev/null
+++ b/intl/locale/rust/oxilangtag-ffi/src/lib.rs
@@ -0,0 +1,126 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use nsstring::nsACString;
+use oxilangtag::LanguageTag;
+
+pub struct LangTag; // Opaque type for ffi interface.
+
+/// Parse a string as a BCP47 language tag. Returns a `LangTag` object if the string is
+/// successfully parsed; this must be freed with `lang_tag_destroy`.
+///
+/// The string `tag` must outlive the `LangTag`.
+///
+/// Returns null if `tag` is not a well-formed BCP47 tag (including if it is not
+/// valid UTF-8).
+#[no_mangle]
+pub extern "C" fn lang_tag_new(tag: &nsACString) -> *mut LangTag {
+    if let Ok(tag_str) = core::str::from_utf8(tag.as_ref()) {
+        if let Ok(language_tag) = LanguageTag::parse(tag_str) {
+            return Box::into_raw(Box::new(language_tag)) as *mut LangTag;
+        }
+    }
+    std::ptr::null_mut()
+}
+
+/// Free a `LangTag` instance.
+#[no_mangle]
+pub extern "C" fn lang_tag_destroy(lang: *mut LangTag) {
+    if lang.is_null() {
+        return;
+    }
+    let _ = unsafe { Box::from_raw(lang as *mut LanguageTag<&str>) };
+}
+
+/// Matches an HTML language attribute against a CSS :lang() selector using the
+/// "extended filtering" algorithm.
+/// The attribute is a BCP47 language tag that was successfully parsed by oxilangtag;
+/// the selector is a string that is treated as a language range per RFC 4647.
+#[no_mangle]
+pub extern "C" fn lang_tag_matches(attribute: *const LangTag, selector: &nsACString) -> bool {
+    // This should only be called with a pointer that we got from lang_tag_new().
+    let lang = unsafe { *(attribute as *const LanguageTag<&str>) };
+
+    // Our callers guarantee that the selector string is valid UTF-8.
+    let range_str = unsafe { selector.as_str_unchecked() };
+
+    if lang.is_empty() || range_str.is_empty() {
+        return false;
+    }
+
+    // RFC 4647 Extended Filtering:
+    // https://datatracker.ietf.org/doc/html/rfc4647#section-3.3.2
+
+    // 1.  Split both the extended language range and the language tag being
+    // compared into a list of subtags by dividing on the hyphen (%x2D)
+    // character.  Two subtags match if either they are the same when
+    // compared case-insensitively or the language range's subtag is the
+    // wildcard '*'.
+
+    let mut range_subtags = range_str.split('-');
+    let mut lang_subtags = lang.as_str().split('-');
+
+    // 2.  Begin with the first subtag in each list.  If the first subtag in
+    // the range does not match the first subtag in the tag, the overall
+    // match fails.  Otherwise, move to the next subtag in both the
+    // range and the tag.
+
+    let mut range_subtag = range_subtags.next();
+    let mut lang_subtag = lang_subtags.next();
+    // Cannot be None, because we checked that both args were non-empty.
+    assert!(range_subtag.is_some() && lang_subtag.is_some());
+    if !(range_subtag.unwrap() == "*"
+        || range_subtag
+            .unwrap()
+            .eq_ignore_ascii_case(lang_subtag.unwrap()))
+    {
+        return false;
+    }
+
+    range_subtag = range_subtags.next();
+    lang_subtag = lang_subtags.next();
+
+    // 3.  While there are more subtags left in the language range's list:
+    loop {
+        // 4.  When the language range's list has no more subtags, the match
+        // succeeds.
+        let Some(range_subtag_str) = range_subtag else {
+            return true;
+        };
+
+        // A.  If the subtag currently being examined in the range is the
+        //     wildcard ('*'), move to the next subtag in the range and
+        //     continue with the loop.
+        if range_subtag_str == "*" {
+            range_subtag = range_subtags.next();
+            continue;
+        }
+
+        // B.  Else, if there are no more subtags in the language tag's
+        //     list, the match fails.
+        let Some(lang_subtag_str) = lang_subtag else {
+            return false;
+        };
+
+        // C.  Else, if the current subtag in the range's list matches the
+        //     current subtag in the language tag's list, move to the next
+        //     subtag in both lists and continue with the loop.
+        if range_subtag_str.eq_ignore_ascii_case(lang_subtag_str) {
+            range_subtag = range_subtags.next();
+            lang_subtag = lang_subtags.next();
+            continue;
+        }
+
+        // D.  Else, if the language tag's subtag is a "singleton" (a single
+        //     letter or digit, which includes the private-use subtag 'x')
+        //     the match fails.
+        if lang_subtag_str.len() == 1 {
+            return false;
+        }
+
+        // E.  Else, move to the next subtag in the language tag's list and
+        //     continue with the loop.
+        lang_subtag = lang_subtags.next();
+    }
+}
diff --git a/intl/locale/rust/unic-langid-ffi/Cargo.toml b/intl/locale/rust/unic-langid-ffi/Cargo.toml
new file mode 100644
index 0000000000..bd969437a6
--- /dev/null
+++ b/intl/locale/rust/unic-langid-ffi/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "unic-langid-ffi"
+version = "0.1.0"
+license = "MPL-2.0"
+authors = ["Zibi Braniecki <zibi@braniecki.net>"]
+edition = "2018"
+
+[dependencies]
+nsstring = { path = "../../../../xpcom/rust/nsstring" }
+thin-vec = { version = "0.2.1", features = ["gecko-ffi"] }
+unic-langid = { version = "0.9", features = ["likelysubtags"] }
diff --git a/intl/locale/rust/unic-langid-ffi/cbindgen.toml b/intl/locale/rust/unic-langid-ffi/cbindgen.toml
new file mode 100644
index 0000000000..3842e5183b
--- /dev/null
+++ b/intl/locale/rust/unic-langid-ffi/cbindgen.toml
@@ -0,0 +1,22 @@
+header = """/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */"""
+autogen_warning = """/* DO NOT MODIFY THIS MANUALLY! This file was generated using cbindgen. See RunCbindgen.py */
+#ifndef mozilla_intl_locale_MozLocaleBindings_h
+#error "Don't include this file directly, instead include MozLocaleBindings.h"
+#endif
+"""
+include_version = true
+braces = "SameLine"
+line_length = 100
+tab_width = 2
+language = "C++"
+namespaces = ["mozilla", "intl", "ffi"]
+
+[parse]
+parse_deps = true
+include = ["unic-langid", "unic-langid-impl"]
+
+[export.rename]
+"ThinVec" = "nsTArray"
+"nsCStr" = "nsDependentCSubstring"
diff --git a/intl/locale/rust/unic-langid-ffi/src/lib.rs b/intl/locale/rust/unic-langid-ffi/src/lib.rs
new file mode 100644
index 0000000000..804a4341e1
--- /dev/null
+++ b/intl/locale/rust/unic-langid-ffi/src/lib.rs
@@ -0,0 +1,168 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use nsstring::{nsACString, nsCStr, nsCString};
+use thin_vec::ThinVec;
+pub use unic_langid::{subtags, CharacterDirection, LanguageIdentifier, LanguageIdentifierError};
+
+pub fn new_langid_for_mozilla(
+    name: &nsACString,
+) -> Result<LanguageIdentifier, LanguageIdentifierError> {
+    if name.eq_ignore_ascii_case(b"ja-jp-mac") {
+        "ja-JP-macos".parse()
+    } else {
+        // Cut out any `.FOO` like `en-US.POSIX`.
+        let mut name: &[u8] = name.as_ref();
+        if let Some(ptr) = name.iter().position(|b| b == &b'.') {
+            name = &name[..ptr];
+        }
+        LanguageIdentifier::from_bytes(name)
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_canonicalize(name: &mut nsACString) -> bool {
+    let langid = new_langid_for_mozilla(name);
+
+    let result = langid.is_ok();
+
+    name.assign(&langid.unwrap_or_default().to_string());
+
+    result
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_new(
+    name: &nsACString,
+    ret_val: &mut bool,
+) -> *mut LanguageIdentifier {
+    let langid = new_langid_for_mozilla(name);
+
+    *ret_val = langid.is_ok();
+    Box::into_raw(Box::new(langid.unwrap_or_default()))
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn unic_langid_destroy(langid: *mut LanguageIdentifier) {
+    let _ = Box::from_raw(langid);
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_as_string(langid: &mut LanguageIdentifier, ret_val: &mut nsACString) {
+    ret_val.assign(&langid.to_string());
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_get_language<'a>(
+    langid: &'a LanguageIdentifier,
+    out: &mut nsCStr<'a>,
+) {
+    *out = nsCStr::from(langid.language.as_str());
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_set_language(
+    langid: &mut LanguageIdentifier,
+    string: &nsACString,
+) -> bool {
+    subtags::Language::from_bytes(string)
+        .map(|lang| langid.language = lang)
+        .is_ok()
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_clear_language(langid: &mut LanguageIdentifier) {
+    langid.language.clear()
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_get_script<'a>(langid: &'a LanguageIdentifier, out: &mut nsCStr<'a>) {
+    *out = nsCStr::from(langid.script.as_ref().map_or("", |s| s.as_str()));
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_set_script(
+    langid: &mut LanguageIdentifier,
+    string: &nsACString,
+) -> bool {
+    subtags::Script::from_bytes(string)
+        .map(|script| langid.script = Some(script))
+        .is_ok()
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_clear_script(langid: &mut LanguageIdentifier) {
+    langid.script = None;
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_get_region<'a>(langid: &'a LanguageIdentifier, out: &mut nsCStr<'a>) {
+    *out = nsCStr::from(langid.region.as_ref().map_or("", |s| s.as_str()));
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_set_region(
+    langid: &mut LanguageIdentifier,
+    string: &nsACString,
+) -> bool {
+    subtags::Region::from_bytes(string)
+        .map(|region| langid.region = Some(region))
+        .is_ok()
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_clear_region(langid: &mut LanguageIdentifier) {
+    langid.region = None;
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_get_variants(
+    langid: &LanguageIdentifier,
+    variants: &mut ThinVec<nsCString>,
+) {
+    for v in langid.variants() {
+        variants.push(v.as_str().into());
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_set_variants(
+    langid: &mut LanguageIdentifier,
+    variants: &ThinVec<nsCString>,
+) -> bool {
+    variants
+        .iter()
+        .map(|v| subtags::Variant::from_bytes(v))
+        .collect::<Result<Vec<_>, _>>()
+        .map(|variants| langid.set_variants(&variants))
+        .is_ok()
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_clear_variants(langid: &mut LanguageIdentifier) {
+    langid.clear_variants()
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_matches(
+    langid: &LanguageIdentifier,
+    other: &LanguageIdentifier,
+    self_as_range: bool,
+    other_as_range: bool,
+) -> bool {
+    langid.matches(other, self_as_range, other_as_range)
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_maximize(langid: &mut LanguageIdentifier) -> bool {
+    langid.maximize()
+}
+
+#[no_mangle]
+pub extern "C" fn unic_langid_is_rtl(name: &nsACString) -> bool {
+    match new_langid_for_mozilla(name) {
+        Ok(langid) => langid.character_direction() == CharacterDirection::RTL,
+        Err(_) => false,
+    }
+}
diff --git a/intl/locale/tests/LangPackMatcherTestUtils.sys.mjs b/intl/locale/tests/LangPackMatcherTestUtils.sys.mjs
new file mode 100644
index 0000000000..4b18f1be13
--- /dev/null
+++ b/intl/locale/tests/LangPackMatcherTestUtils.sys.mjs
@@ -0,0 +1,124 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { LangPackMatcher } from "resource://gre/modules/LangPackMatcher.sys.mjs";
+
+/**
+ * LangPackMatcher.jsm calls out to to the addons store, which involves network requests.
+ * Other tests create a fake addons server, and install mock XPIs. At the time of this
+ * writing that infrastructure is not available for mochitests.
+ *
+ * Instead, this test mocks out APIs that have a side-effect, so the addons of the browser
+ * are never modified.
+ *
+ * The calls to get the app's locale and system's locale are also mocked so that the
+ * different language mismatch scenarios can be run through.
+ *
+ * The locales are BCP 47 identifiers:
+ *
+ * @param {{
+ *   sandbox: SinonSandbox,
+ *   systemLocale: string,
+ *   appLocale, string,
+ * }}
+ */
+export function getAddonAndLocalAPIsMocker(testScope, sandbox) {
+  const { info } = testScope;
+  return function mockAddonAndLocaleAPIs({ systemLocale, appLocale }) {
+    info("Mocking LangPackMatcher.jsm APIs");
+
+    let resolveLangPacks;
+    const langPackPromise = new Promise(resolve => {
+      resolveLangPacks = availableLangpacks => {
+        info(
+          `Resolving which langpacks are available for download: ${JSON.stringify(
+            availableLangpacks
+          )}`
+        );
+        resolve(
+          availableLangpacks.map(locale => ({
+            guid: `langpack-${locale}@firefox.mozilla.org`,
+            type: "language",
+            hash: locale,
+            target_locale: locale,
+            current_compatible_version: {
+              files: [
+                {
+                  platform: "all",
+                  url: `http://example.com/${locale}.langpack.xpi`,
+                },
+              ],
+            },
+          }))
+        );
+      };
+    });
+
+    let resolveInstaller;
+    const installerPromise = new Promise(resolve => {
+      resolveInstaller = () => {
+        info("LangPack install finished.");
+        resolve();
+      };
+    });
+
+    const { mockable } = LangPackMatcher;
+    if (appLocale) {
+      const availableLocales = [appLocale];
+      if (appLocale !== "en-US") {
+        // Ensure the fallback behavior is accurately simulated for Firefox.
+        availableLocales.push("en-US");
+      }
+      sandbox
+        .stub(mockable, "getAvailableLocalesIncludingFallback")
+        .returns(availableLocales);
+      sandbox.stub(mockable, "getDefaultLocale").returns(appLocale);
+      sandbox.stub(mockable, "getAppLocaleAsBCP47").returns(appLocale);
+      sandbox.stub(mockable, "getLastFallbackLocale").returns("en-US");
+    }
+    if (systemLocale) {
+      sandbox.stub(mockable, "getSystemLocale").returns(systemLocale);
+    }
+
+    sandbox.stub(mockable, "getAvailableLangpacks").callsFake(() => {
+      info("Requesting which langpacks are available for download");
+      return langPackPromise;
+    });
+
+    sandbox.stub(mockable, "installLangPack").callsFake(langPack => {
+      info(`LangPack install started, but pending: ${langPack.target_locale}`);
+      return installerPromise;
+    });
+
+    sandbox.stub(mockable, "setRequestedAppLocales").callsFake(locales => {
+      info(
+        `Changing the browser's requested locales to: ${JSON.stringify(
+          locales
+        )}`
+      );
+    });
+
+    return {
+      /**
+       * Resolves the addons API call with available langpacks. Call with a list
+       * of BCP 47 identifiers.
+       *
+       * @type {(availableLangpacks: string[]) => {}}
+       */
+      resolveLangPacks,
+
+      /**
+       * Resolves the pending call to install a langpack.
+       *
+       * @type {() => {}}
+       */
+      resolveInstaller,
+
+      /**
+       * The mocked APIs.
+       */
+      mockable,
+    };
+  };
+}
diff --git a/intl/locale/tests/gtest/TestAppDateTimeFormat.cpp b/intl/locale/tests/gtest/TestAppDateTimeFormat.cpp
new file mode 100644
index 0000000000..a83b373428
--- /dev/null
+++ b/intl/locale/tests/gtest/TestAppDateTimeFormat.cpp
@@ -0,0 +1,310 @@
+#include "gtest/gtest.h"
+#include "mozilla/gtest/MozAssertions.h"
+#include "mozilla/intl/AppDateTimeFormat.h"
+#include "mozilla/intl/DateTimeFormat.h"
+
+namespace mozilla::intl {
+using Style = DateTimeFormat::Style;
+using StyleBag = DateTimeFormat::StyleBag;
+using ComponentsBag = DateTimeFormat::ComponentsBag;
+
+static DateTimeFormat::StyleBag ToStyleBag(Maybe<DateTimeFormat::Style> date,
+                                           Maybe<DateTimeFormat::Style> time) {
+  DateTimeFormat::StyleBag style;
+  style.date = date;
+  style.time = time;
+  return style;
+}
+
+TEST(AppDateTimeFormat, FormatPRExplodedTime)
+{
+  PRTime prTime = 0;
+  PRExplodedTime prExplodedTime;
+  PR_ExplodeTime(prTime, PR_GMTParameters, &prExplodedTime);
+
+  AppDateTimeFormat::sLocale = new nsCString("en-US");
+  AppDateTimeFormat::DeleteCache();
+  StyleBag style = ToStyleBag(Some(Style::Long), Some(Style::Long));
+
+  nsAutoString formattedTime;
+  nsresult rv =
+      AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"January") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1970") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"12:00:00 AM") != kNotFound ||
+              formattedTime.Find(u"12:00:00\u202FAM") != kNotFound ||
+              formattedTime.Find(u"00:00:00") != kNotFound);
+
+  prExplodedTime = {0, 0, 19, 0, 1, 0, 1970, 4, 0, {(19 * 60), 0}};
+
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"January") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1970") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"12:19:00 AM") != kNotFound ||
+              formattedTime.Find(u"12:19:00\u202FAM") != kNotFound ||
+              formattedTime.Find(u"00:19:00") != kNotFound);
+
+  prExplodedTime = {0, 0,    0, 7, 1,
+                    0, 1970, 4, 0, {(6 * 60 * 60), (1 * 60 * 60)}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"January") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1970") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"7:00:00 AM") != kNotFound ||
+              formattedTime.Find(u"7:00:00\u202FAM") != kNotFound ||
+              formattedTime.Find(u"07:00:00") != kNotFound);
+
+  prExplodedTime = {
+      0, 0,    29, 11, 1,
+      0, 1970, 4,  0,  {(10 * 60 * 60) + (29 * 60), (1 * 60 * 60)}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"January") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1970") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"11:29:00 AM") != kNotFound ||
+              formattedTime.Find(u"11:29:00\u202FAM") != kNotFound ||
+              formattedTime.Find(u"11:29:00") != kNotFound);
+
+  prExplodedTime = {0, 0, 37, 23, 31, 11, 1969, 3, 364, {-(23 * 60), 0}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"December") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"31") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1969") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"11:37:00 PM") != kNotFound ||
+              formattedTime.Find(u"11:37:00\u202FPM") != kNotFound ||
+              formattedTime.Find(u"23:37:00") != kNotFound);
+
+  prExplodedTime = {0, 0, 0, 17, 31, 11, 1969, 3, 364, {-(7 * 60 * 60), 0}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"December") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"31") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1969") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"5:00:00 PM") != kNotFound ||
+              formattedTime.Find(u"5:00:00\u202FPM") != kNotFound ||
+              formattedTime.Find(u"17:00:00") != kNotFound);
+
+  prExplodedTime = {
+      0,  0,    47, 14,  31,
+      11, 1969, 3,  364, {-((10 * 60 * 60) + (13 * 60)), (1 * 60 * 60)}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"December") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"31") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1969") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"2:47:00 PM") != kNotFound ||
+              formattedTime.Find(u"2:47:00\u202FPM") != kNotFound ||
+              formattedTime.Find(u"14:47:00") != kNotFound);
+}
+
+TEST(AppDateTimeFormat, DateFormatSelectors)
+{
+  PRTime prTime = 0;
+  PRExplodedTime prExplodedTime;
+  PR_ExplodeTime(prTime, PR_GMTParameters, &prExplodedTime);
+
+  AppDateTimeFormat::sLocale = new nsCString("en-US");
+  AppDateTimeFormat::DeleteCache();
+
+  nsAutoString formattedTime;
+
+  {
+    ComponentsBag components{};
+    components.year = Some(DateTimeFormat::Numeric::Numeric);
+    components.month = Some(DateTimeFormat::Month::TwoDigit);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("01/1970", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+  {
+    ComponentsBag components{};
+    components.year = Some(DateTimeFormat::Numeric::Numeric);
+    components.month = Some(DateTimeFormat::Month::Long);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("January 1970", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+  {
+    ComponentsBag components{};
+    components.month = Some(DateTimeFormat::Month::Long);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("January", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+  {
+    ComponentsBag components{};
+    components.weekday = Some(DateTimeFormat::Text::Short);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("Thu", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+}
+
+TEST(AppDateTimeFormat, FormatPRExplodedTimeForeign)
+{
+  PRTime prTime = 0;
+  PRExplodedTime prExplodedTime;
+  PR_ExplodeTime(prTime, PR_GMTParameters, &prExplodedTime);
+
+  AppDateTimeFormat::sLocale = new nsCString("de-DE");
+  AppDateTimeFormat::DeleteCache();
+  StyleBag style = ToStyleBag(Some(Style::Long), Some(Style::Long));
+
+  nsAutoString formattedTime;
+  nsresult rv =
+      AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"1.") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"Januar") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1970") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"12:00:00 AM") != kNotFound ||
+              formattedTime.Find(u"12:00:00\u202FAM") != kNotFound ||
+              formattedTime.Find(u"00:00:00") != kNotFound);
+
+  prExplodedTime = {0, 0, 19, 0, 1, 0, 1970, 4, 0, {(19 * 60), 0}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"1.") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"Januar") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1970") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"12:19:00 AM") != kNotFound ||
+              formattedTime.Find(u"12:19:00\u202FAM") != kNotFound ||
+              formattedTime.Find(u"00:19:00") != kNotFound);
+
+  prExplodedTime = {0, 0,    0, 7, 1,
+                    0, 1970, 4, 0, {(6 * 60 * 60), (1 * 60 * 60)}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"1.") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"Januar") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1970") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"7:00:00 AM") != kNotFound ||
+              formattedTime.Find(u"7:00:00\u202FAM") != kNotFound ||
+              formattedTime.Find(u"07:00:00") != kNotFound);
+
+  prExplodedTime = {
+      0, 0,    29, 11, 1,
+      0, 1970, 4,  0,  {(10 * 60 * 60) + (29 * 60), (1 * 60 * 60)}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"1.") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"Januar") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1970") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"11:29:00 AM") != kNotFound ||
+              formattedTime.Find(u"11:29:00\u202FAM") != kNotFound ||
+              formattedTime.Find(u"11:29:00") != kNotFound);
+
+  prExplodedTime = {0, 0, 37, 23, 31, 11, 1969, 3, 364, {-(23 * 60), 0}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"31.") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"Dezember") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1969") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"11:37:00 PM") != kNotFound ||
+              formattedTime.Find(u"11:37:00\u202FPM") != kNotFound ||
+              formattedTime.Find(u"23:37:00") != kNotFound);
+
+  prExplodedTime = {0, 0, 0, 17, 31, 11, 1969, 3, 364, {-(7 * 60 * 60), 0}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"31.") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"Dezember") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1969") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"5:00:00 PM") != kNotFound ||
+              formattedTime.Find(u"5:00:00\u202FPM") != kNotFound ||
+              formattedTime.Find(u"17:00:00") != kNotFound);
+
+  prExplodedTime = {
+      0,  0,    47, 14,  31,
+      11, 1969, 3,  364, {-((10 * 60 * 60) + (13 * 60)), (1 * 60 * 60)}};
+  rv = AppDateTimeFormat::Format(style, &prExplodedTime, formattedTime);
+  ASSERT_NS_SUCCEEDED(rv);
+  ASSERT_TRUE(formattedTime.Find(u"31.") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"Dezember") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"1969") != kNotFound);
+  ASSERT_TRUE(formattedTime.Find(u"2:47:00 PM") != kNotFound ||
+              formattedTime.Find(u"2:47:00\u202FPM") != kNotFound ||
+              formattedTime.Find(u"14:47:00") != kNotFound);
+}
+
+TEST(AppDateTimeFormat, DateFormatSelectorsForeign)
+{
+  PRTime prTime = 0;
+  PRExplodedTime prExplodedTime;
+  PR_ExplodeTime(prTime, PR_GMTParameters, &prExplodedTime);
+
+  AppDateTimeFormat::sLocale = new nsCString("de-DE");
+  AppDateTimeFormat::DeleteCache();
+
+  nsAutoString formattedTime;
+  {
+    ComponentsBag components{};
+    components.year = Some(DateTimeFormat::Numeric::Numeric);
+    components.month = Some(DateTimeFormat::Month::TwoDigit);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("01.1970", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+  {
+    ComponentsBag components{};
+    components.year = Some(DateTimeFormat::Numeric::Numeric);
+    components.month = Some(DateTimeFormat::Month::Long);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("Januar 1970", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+  {
+    ComponentsBag components{};
+    components.weekday = Some(DateTimeFormat::Text::Short);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("Do", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+  {
+    ComponentsBag components{};
+    components.weekday = Some(DateTimeFormat::Text::Long);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("Donnerstag", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+  {
+    ComponentsBag components{};
+    components.month = Some(DateTimeFormat::Month::Long);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("Januar", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+  {
+    ComponentsBag components{};
+    components.weekday = Some(DateTimeFormat::Text::Short);
+
+    nsresult rv =
+        AppDateTimeFormat::Format(components, &prExplodedTime, formattedTime);
+    ASSERT_NS_SUCCEEDED(rv);
+    ASSERT_STREQ("Do", NS_ConvertUTF16toUTF8(formattedTime).get());
+  }
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/locale/tests/gtest/TestLocaleService.cpp b/intl/locale/tests/gtest/TestLocaleService.cpp
new file mode 100644
index 0000000000..2cf19727d6
--- /dev/null
+++ b/intl/locale/tests/gtest/TestLocaleService.cpp
@@ -0,0 +1,173 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "gtest/gtest.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/intl/LocaleService.h"
+#include "mozilla/intl/Collator.h"
+
+using namespace mozilla::intl;
+
+TEST(Intl_Locale_LocaleService, CanonicalizeLanguageId)
+{
+  nsCString locale("en-US.POSIX");
+  ASSERT_TRUE(LocaleService::CanonicalizeLanguageId(locale));
+  ASSERT_TRUE(locale.EqualsLiteral("en-US"));
+
+  locale.AssignLiteral("en-US_POSIX");
+  ASSERT_TRUE(LocaleService::CanonicalizeLanguageId(locale));
+  ASSERT_TRUE(locale.EqualsLiteral("en-US-posix"));
+
+  locale.AssignLiteral("en-US-POSIX");
+  ASSERT_TRUE(LocaleService::CanonicalizeLanguageId(locale));
+  ASSERT_TRUE(locale.EqualsLiteral("en-US-posix"));
+
+  locale.AssignLiteral("C");
+  ASSERT_FALSE(LocaleService::CanonicalizeLanguageId(locale));
+  ASSERT_TRUE(locale.EqualsLiteral("und"));
+
+  locale.AssignLiteral("");
+  ASSERT_FALSE(LocaleService::CanonicalizeLanguageId(locale));
+  ASSERT_TRUE(locale.EqualsLiteral("und"));
+}
+
+TEST(Intl_Locale_LocaleService, GetAppLocalesAsBCP47)
+{
+  nsTArray<nsCString> appLocales;
+  LocaleService::GetInstance()->GetAppLocalesAsBCP47(appLocales);
+
+  ASSERT_FALSE(appLocales.IsEmpty());
+}
+
+TEST(Intl_Locale_LocaleService, GetAppLocalesAsLangTags)
+{
+  nsTArray<nsCString> appLocales;
+  LocaleService::GetInstance()->GetAppLocalesAsLangTags(appLocales);
+
+  ASSERT_FALSE(appLocales.IsEmpty());
+}
+
+TEST(Intl_Locale_LocaleService, GetAppLocalesAsLangTags_lastIsPresent)
+{
+  nsAutoCString lastFallbackLocale;
+  LocaleService::GetInstance()->GetLastFallbackLocale(lastFallbackLocale);
+
+  nsTArray<nsCString> appLocales;
+  LocaleService::GetInstance()->GetAppLocalesAsLangTags(appLocales);
+
+  ASSERT_TRUE(appLocales.Contains(lastFallbackLocale));
+}
+
+TEST(Intl_Locale_LocaleService, GetAppLocaleAsLangTag)
+{
+  nsTArray<nsCString> appLocales;
+  LocaleService::GetInstance()->GetAppLocalesAsLangTags(appLocales);
+
+  nsAutoCString locale;
+  LocaleService::GetInstance()->GetAppLocaleAsLangTag(locale);
+
+  ASSERT_TRUE(appLocales[0] == locale);
+}
+
+TEST(Intl_Locale_LocaleService, GetRegionalPrefsLocales)
+{
+  nsTArray<nsCString> rpLocales;
+  LocaleService::GetInstance()->GetRegionalPrefsLocales(rpLocales);
+
+  int32_t len = rpLocales.Length();
+  ASSERT_TRUE(len > 0);
+}
+
+TEST(Intl_Locale_LocaleService, GetWebExposedLocales)
+{
+  const nsTArray<nsCString> spoofLocale{"de"_ns};
+  LocaleService::GetInstance()->SetAvailableLocales(spoofLocale);
+  LocaleService::GetInstance()->SetRequestedLocales(spoofLocale);
+
+  nsTArray<nsCString> pvLocales;
+
+  mozilla::Preferences::SetInt("privacy.spoof_english", 0);
+  LocaleService::GetInstance()->GetWebExposedLocales(pvLocales);
+  ASSERT_TRUE(pvLocales.Length() > 0);
+  ASSERT_TRUE(pvLocales[0].Equals("de"_ns));
+
+  mozilla::Preferences::SetCString("intl.locale.privacy.web_exposed", "zh-TW");
+  LocaleService::GetInstance()->GetWebExposedLocales(pvLocales);
+  ASSERT_TRUE(pvLocales.Length() > 0);
+  ASSERT_TRUE(pvLocales[0].Equals("zh-TW"_ns));
+
+  mozilla::Preferences::SetInt("privacy.spoof_english", 2);
+  LocaleService::GetInstance()->GetWebExposedLocales(pvLocales);
+  ASSERT_EQ(1u, pvLocales.Length());
+  ASSERT_TRUE(pvLocales[0].Equals("en-US"_ns));
+}
+
+TEST(Intl_Locale_LocaleService, GetRequestedLocales)
+{
+  nsTArray<nsCString> reqLocales;
+  LocaleService::GetInstance()->GetRequestedLocales(reqLocales);
+
+  int32_t len = reqLocales.Length();
+  ASSERT_TRUE(len > 0);
+}
+
+TEST(Intl_Locale_LocaleService, GetAvailableLocales)
+{
+  nsTArray<nsCString> availableLocales;
+  LocaleService::GetInstance()->GetAvailableLocales(availableLocales);
+
+  int32_t len = availableLocales.Length();
+  ASSERT_TRUE(len > 0);
+}
+
+TEST(Intl_Locale_LocaleService, GetPackagedLocales)
+{
+  nsTArray<nsCString> packagedLocales;
+  LocaleService::GetInstance()->GetPackagedLocales(packagedLocales);
+
+  int32_t len = packagedLocales.Length();
+  ASSERT_TRUE(len > 0);
+}
+
+TEST(Intl_Locale_LocaleService, GetDefaultLocale)
+{
+  nsAutoCString locStr;
+  LocaleService::GetInstance()->GetDefaultLocale(locStr);
+
+  ASSERT_FALSE(locStr.IsEmpty());
+  Locale loc;
+  ASSERT_TRUE(LocaleParser::TryParse(locStr, loc).isOk());
+}
+
+TEST(Intl_Locale_LocaleService, IsAppLocaleRTL)
+{
+  mozilla::Preferences::SetCString("intl.l10n.pseudo", "bidi");
+  ASSERT_TRUE(LocaleService::GetInstance()->IsAppLocaleRTL());
+  mozilla::Preferences::ClearUser("intl.l10n.pseudo");
+}
+
+TEST(Intl_Locale_LocaleService, TryCreateComponent)
+{
+  {
+    // Create a Collator with the app locale.
+    auto result = LocaleService::GetInstance()->TryCreateComponent<Collator>();
+    ASSERT_TRUE(result.isOk());
+  }
+  {
+    // Create a Collator with the "en" locale.
+    auto result =
+        LocaleService::GetInstance()->TryCreateComponentWithLocale<Collator>(
+            "en");
+    ASSERT_TRUE(result.isOk());
+  }
+  {
+    // Fallback to the app locale when an invalid one is used.
+    auto result =
+        LocaleService::GetInstance()->TryCreateComponentWithLocale<Collator>(
+            "$invalidName");
+    ASSERT_TRUE(result.isOk());
+  }
+}
diff --git a/intl/locale/tests/gtest/TestLocaleServiceNegotiate.cpp b/intl/locale/tests/gtest/TestLocaleServiceNegotiate.cpp
new file mode 100644
index 0000000000..c428e81c8d
--- /dev/null
+++ b/intl/locale/tests/gtest/TestLocaleServiceNegotiate.cpp
@@ -0,0 +1,53 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "gtest/gtest.h"
+#include "mozilla/intl/LocaleService.h"
+
+using namespace mozilla::intl;
+
+TEST(Intl_Locale_LocaleService, Negotiate)
+{
+  nsTArray<nsCString> requestedLocales;
+  nsTArray<nsCString> availableLocales;
+  nsTArray<nsCString> supportedLocales;
+  nsAutoCString defaultLocale("en-US");
+  int32_t strategy = LocaleService::kLangNegStrategyFiltering;
+
+  requestedLocales.AppendElement("sr"_ns);
+
+  availableLocales.AppendElement("sr-Cyrl"_ns);
+  availableLocales.AppendElement("sr-Latn"_ns);
+
+  LocaleService::GetInstance()->NegotiateLanguages(
+      requestedLocales, availableLocales, defaultLocale, strategy,
+      supportedLocales);
+
+  ASSERT_TRUE(supportedLocales.Length() == 2);
+  ASSERT_TRUE(supportedLocales[0].EqualsLiteral("sr-Cyrl"));
+  ASSERT_TRUE(supportedLocales[1].EqualsLiteral("en-US"));
+}
+
+TEST(Intl_Locale_LocaleService, UseLSDefaultLocale)
+{
+  nsTArray<nsCString> requestedLocales;
+  nsTArray<nsCString> availableLocales;
+  nsTArray<nsCString> supportedLocales;
+  nsAutoCString defaultLocale("en-US");
+  int32_t strategy = LocaleService::kLangNegStrategyLookup;
+
+  requestedLocales.AppendElement("sr"_ns);
+
+  availableLocales.AppendElement("de"_ns);
+
+  LocaleService::GetInstance()->NegotiateLanguages(
+      requestedLocales, availableLocales, defaultLocale, strategy,
+      supportedLocales);
+
+  nsAutoCString lsDefaultLocale;
+  LocaleService::GetInstance()->GetDefaultLocale(lsDefaultLocale);
+  ASSERT_TRUE(supportedLocales.Length() == 1);
+  ASSERT_TRUE(supportedLocales[0].Equals(lsDefaultLocale));
+}
diff --git a/intl/locale/tests/gtest/TestOSPreferences.cpp b/intl/locale/tests/gtest/TestOSPreferences.cpp
new file mode 100644
index 0000000000..7e3b71582b
--- /dev/null
+++ b/intl/locale/tests/gtest/TestOSPreferences.cpp
@@ -0,0 +1,205 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "gtest/gtest.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/gtest/MozAssertions.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/intl/LocaleService.h"
+#include "mozilla/intl/OSPreferences.h"
+
+using namespace mozilla::intl;
+
+/**
+ * We test that on all platforms we test against (irrelevant of the tier),
+ * we will be able to retrieve at least a single locale out of the system.
+ *
+ * In theory, that may not be true, but if we encounter such platform we should
+ * decide how to handle this and special case and this test should make
+ * it not happen without us noticing.
+ */
+TEST(Intl_Locale_OSPreferences, GetSystemLocales)
+{
+  nsTArray<nsCString> systemLocales;
+  ASSERT_TRUE(NS_SUCCEEDED(
+      OSPreferences::GetInstance()->GetSystemLocales(systemLocales)));
+
+  ASSERT_FALSE(systemLocales.IsEmpty());
+}
+
+/**
+ * We test that on all platforms we test against (irrelevant of the tier),
+ * we will be able to retrieve at least a single locale out of the system.
+ *
+ * In theory, that may not be true, but if we encounter such platform we should
+ * decide how to handle this and special case and this test should make
+ * it not happen without us noticing.
+ */
+TEST(Intl_Locale_OSPreferences, GetRegionalPrefsLocales)
+{
+  nsTArray<nsCString> rgLocales;
+  ASSERT_TRUE(NS_SUCCEEDED(
+      OSPreferences::GetInstance()->GetRegionalPrefsLocales(rgLocales)));
+
+  ASSERT_FALSE(rgLocales.IsEmpty());
+}
+
+/**
+ * We test that on all platforms we test against,
+ * we will be able to retrieve a date and time pattern.
+ *
+ * This may come back empty on platforms where we don't have platforms
+ * bindings for, so effectively, we're testing for crashes. We should
+ * never crash.
+ */
+TEST(Intl_Locale_OSPreferences, GetDateTimePattern)
+{
+  nsAutoCString pattern;
+  OSPreferences* osprefs = OSPreferences::GetInstance();
+
+  struct Test {
+    int dateStyle;
+    int timeStyle;
+    const char* locale;
+  };
+  Test tests[] = {{0, 0, ""},   {1, 0, "pl"}, {2, 0, "de-DE"}, {3, 0, "fr"},
+                  {4, 0, "ar"},
+
+                  {0, 1, ""},   {0, 2, "it"}, {0, 3, ""},      {0, 4, "ru"},
+
+                  {4, 1, ""},   {3, 2, "cs"}, {2, 3, ""},      {1, 4, "ja"}};
+
+  for (unsigned i = 0; i < mozilla::ArrayLength(tests); i++) {
+    const Test& t = tests[i];
+    if (NS_SUCCEEDED(osprefs->GetDateTimePattern(
+            t.dateStyle, t.timeStyle, nsDependentCString(t.locale), pattern))) {
+      ASSERT_TRUE((t.dateStyle == 0 && t.timeStyle == 0) || !pattern.IsEmpty());
+    }
+  }
+
+  // If the locale is not specified, we should get the pattern corresponding to
+  // the first regional prefs locale.
+  AutoTArray<nsCString, 10> rpLocales;
+  LocaleService::GetInstance()->GetRegionalPrefsLocales(rpLocales);
+  ASSERT_TRUE(rpLocales.Length() > 0);
+
+  nsAutoCString rpLocalePattern;
+  ASSERT_TRUE(NS_SUCCEEDED(
+      osprefs->GetDateTimePattern(mozIOSPreferences::dateTimeFormatStyleLong,
+                                  mozIOSPreferences::dateTimeFormatStyleLong,
+                                  rpLocales[0], rpLocalePattern)));
+  ASSERT_TRUE(NS_SUCCEEDED(
+      osprefs->GetDateTimePattern(mozIOSPreferences::dateTimeFormatStyleLong,
+                                  mozIOSPreferences::dateTimeFormatStyleLong,
+                                  nsDependentCString(""), pattern)));
+  ASSERT_EQ(rpLocalePattern, pattern);
+}
+
+/**
+ * Test that is possible to override the OS defaults through a pref.
+ */
+TEST(Intl_Locale_OSPreferences, GetDateTimePatternPrefOverrides)
+{
+  nsresult nr;
+  nsAutoCString default_pattern, pattern;
+  OSPreferences* osprefs = OSPreferences::GetInstance();
+
+  struct {
+    const char* DatePref;
+    const char* TimePref;
+    int32_t DateTimeFormatStyle;
+  } configs[] = {{"intl.date_time.pattern_override.date_short",
+                  "intl.date_time.pattern_override.time_short",
+                  mozIOSPreferences::dateTimeFormatStyleShort},
+                 {"intl.date_time.pattern_override.date_medium",
+                  "intl.date_time.pattern_override.time_medium",
+                  mozIOSPreferences::dateTimeFormatStyleMedium},
+                 {"intl.date_time.pattern_override.date_long",
+                  "intl.date_time.pattern_override.time_long",
+                  mozIOSPreferences::dateTimeFormatStyleLong},
+                 {"intl.date_time.pattern_override.date_full",
+                  "intl.date_time.pattern_override.time_full",
+                  mozIOSPreferences::dateTimeFormatStyleFull}};
+
+  for (const auto& config : configs) {
+    // Get default value for the OS
+    nr = osprefs->GetDateTimePattern(config.DateTimeFormatStyle,
+                                     mozIOSPreferences::dateTimeFormatStyleNone,
+                                     nsDependentCString(""), default_pattern);
+    ASSERT_NS_SUCCEEDED(nr);
+
+    // Override date format
+    mozilla::Preferences::SetCString(config.DatePref, "yy-MM");
+    nr = osprefs->GetDateTimePattern(config.DateTimeFormatStyle,
+                                     mozIOSPreferences::dateTimeFormatStyleNone,
+                                     nsDependentCString(""), pattern);
+    ASSERT_NS_SUCCEEDED(nr);
+    ASSERT_TRUE(pattern.EqualsASCII("yy-MM"));
+
+    // Override time format
+    mozilla::Preferences::SetCString(config.TimePref, "HH:mm");
+    nr = osprefs->GetDateTimePattern(mozIOSPreferences::dateTimeFormatStyleNone,
+                                     config.DateTimeFormatStyle,
+                                     nsDependentCString(""), pattern);
+    ASSERT_NS_SUCCEEDED(nr);
+    ASSERT_TRUE(pattern.EqualsASCII("HH:mm"));
+
+    // Override both
+    nr = osprefs->GetDateTimePattern(config.DateTimeFormatStyle,
+                                     config.DateTimeFormatStyle,
+                                     nsDependentCString(""), pattern);
+    ASSERT_NS_SUCCEEDED(nr);
+    ASSERT_TRUE(pattern.Find("yy-MM") != kNotFound);
+    ASSERT_TRUE(pattern.Find("HH:mm") != kNotFound);
+
+    // Clear overrides, we should get the default value back.
+    mozilla::Preferences::ClearUser(config.DatePref);
+    mozilla::Preferences::ClearUser(config.TimePref);
+    nr = osprefs->GetDateTimePattern(config.DateTimeFormatStyle,
+                                     mozIOSPreferences::dateTimeFormatStyleNone,
+                                     nsDependentCString(""), pattern);
+    ASSERT_NS_SUCCEEDED(nr);
+    ASSERT_EQ(default_pattern, pattern);
+  }
+
+  // Test overriding connector
+  nr = osprefs->GetDateTimePattern(mozIOSPreferences::dateTimeFormatStyleShort,
+                                   mozIOSPreferences::dateTimeFormatStyleShort,
+                                   nsDependentCString(""), default_pattern);
+
+  mozilla::Preferences::SetCString("intl.date_time.pattern_override.date_short",
+                                   "yyyy-MM-dd");
+  mozilla::Preferences::SetCString("intl.date_time.pattern_override.time_short",
+                                   "HH:mm:ss");
+  mozilla::Preferences::SetCString(
+      "intl.date_time.pattern_override.connector_short", "{1} {0}");
+  nr = osprefs->GetDateTimePattern(mozIOSPreferences::dateTimeFormatStyleShort,
+                                   mozIOSPreferences::dateTimeFormatStyleShort,
+                                   nsDependentCString(""), pattern);
+  ASSERT_NS_SUCCEEDED(nr);
+  ASSERT_TRUE(pattern.EqualsASCII("yyyy-MM-dd HH:mm:ss"));
+
+  // Reset to date and time to defaults
+  mozilla::Preferences::ClearUser("intl.date_time.pattern_override.date_short");
+  mozilla::Preferences::ClearUser("intl.date_time.pattern_override.time_short");
+
+  // Invalid patterns are ignored
+  mozilla::Preferences::SetCString(
+      "intl.date_time.pattern_override.connector_short", "hello, world!");
+  nr = osprefs->GetDateTimePattern(mozIOSPreferences::dateTimeFormatStyleShort,
+                                   mozIOSPreferences::dateTimeFormatStyleShort,
+                                   nsDependentCString(""), pattern);
+  ASSERT_NS_SUCCEEDED(nr);
+  ASSERT_EQ(default_pattern, pattern);
+
+  // Clearing the override results in getting the default pattern back.
+  mozilla::Preferences::ClearUser(
+      "intl.date_time.pattern_override.connector_short");
+  nr = osprefs->GetDateTimePattern(mozIOSPreferences::dateTimeFormatStyleShort,
+                                   mozIOSPreferences::dateTimeFormatStyleShort,
+                                   nsDependentCString(""), pattern);
+  ASSERT_NS_SUCCEEDED(nr);
+  ASSERT_EQ(default_pattern, pattern);
+}
diff --git a/intl/locale/tests/gtest/moz.build b/intl/locale/tests/gtest/moz.build
new file mode 100644
index 0000000000..bd2a2b8f86
--- /dev/null
+++ b/intl/locale/tests/gtest/moz.build
@@ -0,0 +1,14 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+UNIFIED_SOURCES += [
+    "TestAppDateTimeFormat.cpp",
+    "TestLocaleService.cpp",
+    "TestLocaleServiceNegotiate.cpp",
+    "TestOSPreferences.cpp",
+]
+
+FINAL_LIBRARY = "xul-gtest"
diff --git a/intl/locale/tests/sort/us-ascii_base.txt b/intl/locale/tests/sort/us-ascii_base.txt
new file mode 100644
index 0000000000..3bc7fbc7be
--- /dev/null
+++ b/intl/locale/tests/sort/us-ascii_base.txt
@@ -0,0 +1,95 @@
+ 
+!
+"
+#
+$
+%
+&
+'
+(
+)
+*
++
+,
+-
+.
+/
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
+:
+;
+<
+=
+>
+?
+@
+A
+B
+C
+D
+E
+F
+G
+H
+I
+J
+K
+L
+M
+N
+O
+P
+Q
+R
+S
+T
+U
+V
+W
+X
+Y
+Z
+[
+\
+]
+^
+_
+`
+a
+b
+c
+d
+e
+f
+g
+h
+i
+j
+k
+l
+m
+n
+o
+p
+q
+r
+s
+t
+u
+v
+w
+x
+y
+z
+{
+|
+}
+~
diff --git a/intl/locale/tests/sort/us-ascii_base_case_res.txt b/intl/locale/tests/sort/us-ascii_base_case_res.txt
new file mode 100644
index 0000000000..3e526d226b
--- /dev/null
+++ b/intl/locale/tests/sort/us-ascii_base_case_res.txt
@@ -0,0 +1,96 @@
+'
+-
+ 
+!
+"
+#
+$
+%
+&
+(
+)
+*
+,
+.
+/
+:
+;
+?
+@
+[
+\
+]
+^
+_
+`
+{
+|
+}
+~
++
+<
+=
+>
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
+a
+A
+b
+B
+c
+C
+d
+D
+e
+E
+f
+F
+g
+G
+h
+H
+i
+I
+j
+J
+k
+K
+l
+L
+m
+M
+n
+N
+o
+O
+p
+P
+q
+Q
+r
+R
+s
+S
+t
+T
+u
+U
+v
+V
+w
+W
+x
+X
+y
+Y
+z
+Z
+
diff --git a/intl/locale/tests/sort/us-ascii_base_nocase_res.txt b/intl/locale/tests/sort/us-ascii_base_nocase_res.txt
new file mode 100644
index 0000000000..2d18096db5
--- /dev/null
+++ b/intl/locale/tests/sort/us-ascii_base_nocase_res.txt
@@ -0,0 +1,96 @@
+'
+-
+ 
+!
+"
+#
+$
+%
+&
+(
+)
+*
+,
+.
+/
+:
+;
+?
+@
+[
+\
+]
+^
+_
+`
+{
+|
+}
+~
++
+<
+=
+>
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
+A
+a
+B
+b
+C
+c
+d
+D
+e
+E
+f
+F
+g
+G
+h
+H
+i
+I
+j
+J
+k
+K
+L
+l
+m
+M
+n
+N
+O
+o
+p
+P
+q
+Q
+r
+R
+s
+S
+t
+T
+u
+U
+v
+V
+w
+W
+X
+x
+Y
+y
+Z
+z
+
diff --git a/intl/locale/tests/sort/us-ascii_sort.txt b/intl/locale/tests/sort/us-ascii_sort.txt
new file mode 100644
index 0000000000..b6962f8de8
--- /dev/null
+++ b/intl/locale/tests/sort/us-ascii_sort.txt
@@ -0,0 +1,78 @@
+ludwig van beethoven
+Ludwig van Beethoven
+Ludwig van beethoven
+Jane
+jane
+JANE
+jAne
+jaNe
+janE
+JAne
+JaNe
+JanE
+JANe
+JaNE
+JAnE
+jANE
+Umberto Eco
+Umberto eco
+umberto eco
+umberto Eco
+UMBERTO ECO
+ace
+bash
+*ace
+!ace
+%ace
+~ace
+#ace
+cork
+denizen
+[denizen]
+(denizen)
+{denizen}
+/denizen/
+#denizen#
+$denizen$
+@denizen
+elf
+full
+gnome
+gnomic investigation of typological factors in the grammaticalization process of Malayo-Polynesian substaratum in the protoAltaic vocabulary core in the proto-layers of pre-historic Japanese
+gnomic investigation of typological factors in the grammaticalization process of Malayo-Polynesian substaratum in the protoAltaic vocabulary core in the proto-layers of pre-historic Javanese
+hint
+Internationalization
+Zinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalization
+Zinternationalization internationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizatio
+n
+Zinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalization internationalizationinternationalizationinternationalizationinternationalizationinternationalization
+ZinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizaTioninternationalizationinternationalizationinternationalizationinternationalization
+ZinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizaTion
+jostle
+kin
+Laymen
+lumens
+Lumens
+motleycrew
+motley crew
+niven's creative talents
+nivens creative talents
+opie
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the Rockies
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the Rokkies
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the rockies
+quilt's
+quilts
+quilt
+Rondo
+street
+tamale oxidization and iodization in rapid progress
+tamale oxidization and iodization in rapid Progress
+until
+vera
+Wobble
+Xanadu's legenary imaginary floccinaucinihilipilification in localization of theoretical portions of glottochronological understanding of the phoneme
+Xanadu's legenary imaginary floccinaucinihilipilification in localization of theoretical portions of glottochronological understanding of the phoname
+yearn
+zodiac
+
diff --git a/intl/locale/tests/sort/us-ascii_sort_case_res.txt b/intl/locale/tests/sort/us-ascii_sort_case_res.txt
new file mode 100644
index 0000000000..2720a83426
--- /dev/null
+++ b/intl/locale/tests/sort/us-ascii_sort_case_res.txt
@@ -0,0 +1,79 @@
+!ace
+#ace
+#denizen#
+$denizen$
+%ace
+(denizen)
+*ace
+/denizen/
+@denizen
+[denizen]
+{denizen}
+~ace
+ace
+bash
+cork
+denizen
+elf
+full
+gnome
+gnomic investigation of typological factors in the grammaticalization process of Malayo-Polynesian substaratum in the protoAltaic vocabulary core in the proto-layers of pre-historic Japanese
+gnomic investigation of typological factors in the grammaticalization process of Malayo-Polynesian substaratum in the protoAltaic vocabulary core in the proto-layers of pre-historic Javanese
+hint
+Internationalization
+jane
+janE
+jaNe
+jAne
+jANE
+Jane
+JanE
+JaNe
+JaNE
+JAne
+JAnE
+JANe
+JANE
+jostle
+kin
+Laymen
+ludwig van beethoven
+Ludwig van beethoven
+Ludwig van Beethoven
+lumens
+Lumens
+motley crew
+motleycrew
+nivens creative talents
+niven's creative talents
+opie
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the rockies
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the Rockies
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the Rokkies
+quilt
+quilts
+quilt's
+Rondo
+street
+tamale oxidization and iodization in rapid progress
+tamale oxidization and iodization in rapid Progress
+umberto eco
+umberto Eco
+Umberto eco
+Umberto Eco
+UMBERTO ECO
+until
+veda
+Veda
+vera
+Vera
+Wobble
+Xanadu's legenary imaginary floccinaucinihilipilification in localization of theoretical portions of glottochronological understanding of the phoname
+Xanadu's legenary imaginary floccinaucinihilipilification in localization of theoretical portions of glottochronological understanding of the phoneme
+yearn
+Zinternationalization internationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalization
+Zinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalization internationalizationinternationalizationinternationalizationinternationalizationinternationalization
+Zinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalization
+ZinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizaTion
+ZinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizaTioninternationalizationinternationalizationinternationalizationinternationalization
+zodiac
diff --git a/intl/locale/tests/sort/us-ascii_sort_nocase_res.txt b/intl/locale/tests/sort/us-ascii_sort_nocase_res.txt
new file mode 100644
index 0000000000..fa3426d4e9
--- /dev/null
+++ b/intl/locale/tests/sort/us-ascii_sort_nocase_res.txt
@@ -0,0 +1,79 @@
+!ace
+#ace
+#denizen#
+$denizen$
+%ace
+(denizen)
+*ace
+/denizen/
+@denizen
+[denizen]
+{denizen}
+~ace
+ace
+bash
+cork
+denizen
+elf
+full
+gnome
+gnomic investigation of typological factors in the grammaticalization process of Malayo-Polynesian substaratum in the protoAltaic vocabulary core in the proto-layers of pre-historic Japanese
+gnomic investigation of typological factors in the grammaticalization process of Malayo-Polynesian substaratum in the protoAltaic vocabulary core in the proto-layers of pre-historic Javanese
+hint
+Internationalization
+JANe
+jANE
+JAnE
+JaNE
+Jane
+JanE
+JaNe
+JAne
+janE
+jaNe
+jAne
+JANE
+jane
+jostle
+kin
+Laymen
+Ludwig van beethoven
+Ludwig van Beethoven
+ludwig van beethoven
+Lumens
+lumens
+motley crew
+motleycrew
+nivens creative talents
+niven's creative talents
+opie
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the rockies
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the Rockies
+posh restaurants in surbanized and still urban incorporated subsection of this beautifl city in the Rokkies
+quilt
+quilts
+quilt's
+Rondo
+street
+tamale oxidization and iodization in rapid Progress
+tamale oxidization and iodization in rapid progress
+Umberto eco
+Umberto Eco
+umberto eco
+UMBERTO ECO
+umberto Eco
+until
+Veda
+veda
+Vera
+vera
+Wobble
+Xanadu's legenary imaginary floccinaucinihilipilification in localization of theoretical portions of glottochronological understanding of the phoname
+Xanadu's legenary imaginary floccinaucinihilipilification in localization of theoretical portions of glottochronological understanding of the phoneme
+yearn
+Zinternationalization internationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalization
+Zinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalization internationalizationinternationalizationinternationalizationinternationalizationinternationalization
+ZinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizaTioninternationalizationinternationalizationinternationalizationinternationalization
+ZinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizaTion
+Zinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalizationinternationalization
+zodiac
diff --git a/intl/locale/tests/unit/data/chrome.manifest b/intl/locale/tests/unit/data/chrome.manifest
new file mode 100644
index 0000000000..a8678deef3
--- /dev/null
+++ b/intl/locale/tests/unit/data/chrome.manifest
@@ -0,0 +1 @@
+content locale ./
diff --git a/intl/locale/tests/unit/data/intl_on_workers_worker.js b/intl/locale/tests/unit/data/intl_on_workers_worker.js
new file mode 100644
index 0000000000..e6f01e71a8
--- /dev/null
+++ b/intl/locale/tests/unit/data/intl_on_workers_worker.js
@@ -0,0 +1,6 @@
+/* eslint-env worker */
+
+self.onmessage = function (data) {
+  let myLocale = Intl.NumberFormat().resolvedOptions().locale;
+  self.postMessage(myLocale);
+};
diff --git a/intl/locale/tests/unit/test_bug22310.js b/intl/locale/tests/unit/test_bug22310.js
new file mode 100644
index 0000000000..c14296d931
--- /dev/null
+++ b/intl/locale/tests/unit/test_bug22310.js
@@ -0,0 +1,65 @@
+String.prototype.has = function (s) {
+  return this.includes(s);
+};
+
+function dt(locale) {
+  var date = new Date("2008-06-30T13:56:34");
+  const dtOptions = {
+    year: "numeric",
+    month: "long",
+    day: "numeric",
+    hour: "numeric",
+    minute: "numeric",
+    second: "numeric",
+  };
+  return date.toLocaleString(locale, dtOptions);
+}
+
+var all_passed = true;
+const tests = [
+  [dt("en-US").has("June"), "month name in en-US"],
+  [dt("en-US").has("2008"), "year in en-US"],
+  [dt("da").has("jun"), "month name in da"],
+  [dt("da-DK") == dt("da"), "da same as da-DK"],
+  [
+    dt("en-GB").has("30") &&
+      dt("en-GB").has("June") &&
+      dt("en-GB").indexOf("30") < dt("en-GB").indexOf("June"),
+    "day before month in en-GB",
+  ],
+  [
+    dt("en-US").has("30") &&
+      dt("en-US").has("June") &&
+      dt("en-US").indexOf("30") > dt("en-US").indexOf("June"),
+    "month before day in en-US",
+  ],
+  [dt("ja-JP").has("\u5E746\u670830\u65E5"), "year month and day in ja-JP"],
+  // The Firefox locale code ja-JP-mac will be resolved to a BCP47-compliant
+  // tag ja-JP-x-lvariant-mac by uloc_toLanguageTag
+  [
+    dt("ja-JP") == dt("ja-JP-x-lvariant-mac"),
+    "ja-JP-x-lvariant-mac same as ja-JP",
+  ],
+  [dt("nn-NO").has("juni"), "month name in nn-NO"],
+  [dt("nb-NO").has("juni"), "month name in nb-NO"],
+  // Bug 1261775 - failures on win10
+  //[dt("no-NO").has("30. juni"), "month name in no-NO"],
+  [dt("sv-SE").has("30 jun"), "month name in sv-SE"],
+  [dt("kok").has("\u091C\u0942\u0928"), "month name in kok"],
+  [dt("ta-IN").has("\u0B9C\u0BC2\u0BA9\u0BCD"), "month name in ta-IN"],
+  [!!dt("ab-CD").length, "fallback for ab-CD"],
+];
+
+function one_test(testcase, msg) {
+  if (!testcase) {
+    all_passed = false;
+    dump("Unexpected date format: " + msg + "\n");
+  }
+}
+
+function run_test() {
+  for (var i = 0; i < tests.length; ++i) {
+    one_test(tests[i][0], tests[i][1]);
+  }
+  Assert.ok(all_passed);
+}
diff --git a/intl/locale/tests/unit/test_intl_on_workers.js b/intl/locale/tests/unit/test_intl_on_workers.js
new file mode 100644
index 0000000000..b5e05c4678
--- /dev/null
+++ b/intl/locale/tests/unit/test_intl_on_workers.js
@@ -0,0 +1,29 @@
+function run_test() {
+  do_load_manifest("data/chrome.manifest");
+
+  if (typeof Intl !== "object") {
+    dump("Intl not enabled, skipping test\n");
+    equal(true, true);
+    return;
+  }
+
+  let mainThreadLocale = Intl.NumberFormat().resolvedOptions().locale;
+  let testWorker = new Worker(
+    "chrome://locale/content/intl_on_workers_worker.js"
+  );
+  testWorker.onmessage = function (e) {
+    try {
+      let workerLocale = e.data;
+      equal(
+        mainThreadLocale,
+        workerLocale,
+        "Worker should inherit Intl locale from main thread."
+      );
+    } finally {
+      do_test_finished();
+    }
+  };
+
+  do_test_pending();
+  testWorker.postMessage("go!");
+}
diff --git a/intl/locale/tests/unit/test_langPackMatcher.js b/intl/locale/tests/unit/test_langPackMatcher.js
new file mode 100644
index 0000000000..03adec7bff
--- /dev/null
+++ b/intl/locale/tests/unit/test_langPackMatcher.js
@@ -0,0 +1,287 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const { getAddonAndLocalAPIsMocker } = ChromeUtils.importESModule(
+  "resource://testing-common/LangPackMatcherTestUtils.sys.mjs"
+);
+const { LangPackMatcher } = ChromeUtils.importESModule(
+  "resource://gre/modules/LangPackMatcher.sys.mjs"
+);
+const { sinon } = ChromeUtils.importESModule(
+  "resource://testing-common/Sinon.sys.mjs"
+);
+
+const sandbox = sinon.createSandbox();
+const mockAddonAndLocaleAPIs = getAddonAndLocalAPIsMocker(this, sandbox);
+
+add_task(function initSandbox() {
+  registerCleanupFunction(() => {
+    sandbox.restore();
+  });
+});
+
+add_task(function test_appLocaleLanguageMismatch() {
+  sandbox.restore();
+  mockAddonAndLocaleAPIs({
+    systemLocale: "es-ES",
+    appLocale: "en-US",
+  });
+
+  deepEqual(LangPackMatcher.getAppAndSystemLocaleInfo(), {
+    systemLocaleRaw: "es-ES",
+    systemLocale: { baseName: "es-ES", language: "es", region: "ES" },
+    appLocaleRaw: "en-US",
+    appLocale: { baseName: "en-US", language: "en", region: "US" },
+    matchType: "language-mismatch",
+    canLiveReload: true,
+    displayNames: {
+      systemLanguage: "Español (ES)",
+      appLanguage: "English (US)",
+    },
+  });
+});
+
+add_task(function test_appLocaleRegionMismatch() {
+  sandbox.restore();
+  mockAddonAndLocaleAPIs({
+    sandbox,
+    systemLocale: "en-CA",
+    appLocale: "en-US",
+  });
+
+  deepEqual(LangPackMatcher.getAppAndSystemLocaleInfo(), {
+    systemLocaleRaw: "en-CA",
+    systemLocale: { baseName: "en-CA", language: "en", region: "CA" },
+    appLocaleRaw: "en-US",
+    appLocale: { baseName: "en-US", language: "en", region: "US" },
+    matchType: "region-mismatch",
+    canLiveReload: true,
+    displayNames: {
+      systemLanguage: "English (CA)",
+      appLanguage: "English (US)",
+    },
+  });
+});
+
+add_task(function test_appLocaleScriptMismatch() {
+  sandbox.restore();
+  // Script mismatch:
+  mockAddonAndLocaleAPIs({
+    sandbox,
+    systemLocale: "zh-Hans-CN",
+    appLocale: "zh-CN",
+  });
+
+  deepEqual(LangPackMatcher.getAppAndSystemLocaleInfo(), {
+    systemLocaleRaw: "zh-Hans-CN",
+    systemLocale: { baseName: "zh-Hans-CN", language: "zh", region: "CN" },
+    appLocaleRaw: "zh-CN",
+    appLocale: { baseName: "zh-CN", language: "zh", region: "CN" },
+    matchType: "match",
+    canLiveReload: true,
+    displayNames: {
+      systemLanguage: "Chinese (Hans, China)",
+      appLanguage: "简体中文",
+    },
+  });
+});
+
+add_task(function test_appLocaleInvalidSystem() {
+  sandbox.restore();
+  // Script mismatch:
+  mockAddonAndLocaleAPIs({
+    sandbox,
+    systemLocale: "Not valid",
+    appLocale: "en-US",
+  });
+
+  deepEqual(LangPackMatcher.getAppAndSystemLocaleInfo(), {
+    systemLocaleRaw: "Not valid",
+    systemLocale: null,
+    appLocaleRaw: "en-US",
+    appLocale: { baseName: "en-US", language: "en", region: "US" },
+    matchType: "unknown",
+    canLiveReload: null,
+    displayNames: { systemLanguage: null, appLanguage: "English (US)" },
+  });
+});
+
+add_task(function test_bidiSwitchDisabled() {
+  Services.prefs.setBoolPref(
+    "intl.multilingual.liveReloadBidirectional",
+    false
+  );
+  sandbox.restore();
+  // Script mismatch:
+  mockAddonAndLocaleAPIs({
+    sandbox,
+    systemLocale: "ar-EG",
+    appLocale: "en-US",
+  });
+
+  deepEqual(LangPackMatcher.getAppAndSystemLocaleInfo(), {
+    systemLocaleRaw: "ar-EG",
+    systemLocale: { baseName: "ar-EG", language: "ar", region: "EG" },
+    appLocaleRaw: "en-US",
+    appLocale: { baseName: "en-US", language: "en", region: "US" },
+    matchType: "language-mismatch",
+    canLiveReload: false,
+    displayNames: {
+      systemLanguage: "Arabic (Egypt)",
+      appLanguage: "English (US)",
+    },
+  });
+  Services.prefs.clearUserPref("intl.multilingual.liveReloadBidirectional");
+});
+
+add_task(async function test_bidiSwitchEnabled() {
+  Services.prefs.setBoolPref("intl.multilingual.liveReloadBidirectional", true);
+  sandbox.restore();
+  // Script mismatch:
+  mockAddonAndLocaleAPIs({
+    sandbox,
+    systemLocale: "ar-EG",
+    appLocale: "en-US",
+  });
+
+  deepEqual(LangPackMatcher.getAppAndSystemLocaleInfo(), {
+    systemLocaleRaw: "ar-EG",
+    systemLocale: { baseName: "ar-EG", language: "ar", region: "EG" },
+    appLocaleRaw: "en-US",
+    appLocale: { baseName: "en-US", language: "en", region: "US" },
+    matchType: "language-mismatch",
+    canLiveReload: true,
+    displayNames: {
+      systemLanguage: "Arabic (Egypt)",
+      appLanguage: "English (US)",
+    },
+  });
+
+  Services.prefs.clearUserPref("intl.multilingual.liveReloadBidirectional");
+});
+
+function shuffle(array) {
+  return array
+    .map(value => ({ value, sort: Math.random() }))
+    .sort((a, b) => a.sort - b.sort)
+    .map(({ value }) => value);
+}
+
+add_task(async function test_negotiateLangPacks() {
+  const negotiations = [
+    {
+      // Exact match found.
+      systemLocale: "en-US",
+      availableLangPacks: ["en", "en-US", "zh", "zh-CN", "zh-Hans-CN"],
+      expectedLangPack: "en-US",
+      expectedDisplayName: "English (US)",
+    },
+    {
+      // Region-less match.
+      systemLocale: "en-CA",
+      availableLangPacks: ["en", "en-US", "zh", "zh-CN", "zh-Hans-CN"],
+      expectedLangPack: "en",
+      expectedDisplayName: "English",
+    },
+    {
+      // Fallback to a different region.
+      systemLocale: "en-CA",
+      availableLangPacks: ["en-US", "zh", "zh-CN", "zh-Hans-CN"],
+      expectedLangPack: "en-US",
+      expectedDisplayName: "English (US)",
+    },
+    {
+      // Match with a script. zh-Hans-CN is the locale used with simplified
+      // Chinese scripts, while zh-CN uses the Latin script.
+      systemLocale: "zh-Hans-CN",
+      availableLangPacks: ["en", "en-US", "zh", "zh-CN", "zh-TW", "zh-Hans-CN"],
+      expectedLangPack: "zh-Hans-CN",
+      expectedDisplayName: "Chinese (Hans, China)",
+    },
+    {
+      // Match excluding script but matching region. zh-Hant-TW should
+      // match zh-TW before zh-CN.
+      systemLocale: "zh-Hant-TW",
+      availableLangPacks: ["en", "zh", "zh-CN", "zh-TW"],
+      expectedLangPack: "zh-TW",
+      expectedDisplayName: "正體中文",
+    },
+    {
+      // No reasonable match could be found.
+      systemLocale: "tlh", // Klingon
+      availableLangPacks: ["en", "en-US", "zh", "zh-CN", "zh-Hans-CN"],
+      expectedLangPack: null,
+      expectedDisplayName: null,
+    },
+    {
+      // Weird, but valid locale identifiers.
+      systemLocale: "en-US-u-hc-h23-ca-islamic-civil-ss-true",
+      availableLangPacks: ["en", "en-US", "zh", "zh-CN", "zh-Hans-CN"],
+      expectedLangPack: "en-US",
+      expectedDisplayName: "English (US)",
+    },
+    {
+      // Invalid system locale
+      systemLocale: "Not valid",
+      availableLangPacks: ["en", "en-US", "zh", "zh-CN", "zh-Hans-CN"],
+      expectedLangPack: null,
+      expectedDisplayName: null,
+    },
+  ];
+
+  for (const {
+    systemLocale,
+    availableLangPacks,
+    expectedLangPack,
+    expectedDisplayName,
+  } of negotiations) {
+    sandbox.restore();
+    const { resolveLangPacks } = mockAddonAndLocaleAPIs({
+      sandbox,
+      systemLocale,
+    });
+
+    const promise = LangPackMatcher.negotiateLangPackForLanguageMismatch();
+    // Shuffle the order to ensure that this test doesn't require on ordering of the
+    // langpack responses.
+    resolveLangPacks(shuffle(availableLangPacks));
+    const { langPack, langPackDisplayName } = await promise;
+    equal(
+      langPack?.target_locale,
+      expectedLangPack,
+      `Resolve the systemLocale "${systemLocale}" with available langpacks: ${JSON.stringify(
+        availableLangPacks
+      )}`
+    );
+    equal(
+      langPackDisplayName,
+      expectedDisplayName,
+      "The display name matches."
+    );
+  }
+});
+
+add_task(async function test_ensureLangPackInstalled() {
+  sandbox.restore();
+  const { resolveLangPacks, resolveInstaller } = mockAddonAndLocaleAPIs({
+    sandbox,
+    systemLocale: "es-ES",
+    appLocale: "en-US",
+  });
+
+  const negotiatePromise =
+    LangPackMatcher.negotiateLangPackForLanguageMismatch();
+  resolveLangPacks(["es-ES"]);
+  const { langPack } = await negotiatePromise;
+
+  const installPromise1 = LangPackMatcher.ensureLangPackInstalled(langPack);
+  const installPromise2 = LangPackMatcher.ensureLangPackInstalled(langPack);
+
+  resolveInstaller(["fake langpack"]);
+
+  info("Ensure both installers resolve when called twice in a row.");
+  await installPromise1;
+  await installPromise2;
+  ok(true, "Both were called.");
+});
diff --git a/intl/locale/tests/unit/test_localeService.js b/intl/locale/tests/unit/test_localeService.js
new file mode 100644
index 0000000000..ae2d949e80
--- /dev/null
+++ b/intl/locale/tests/unit/test_localeService.js
@@ -0,0 +1,240 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const osPrefs = Cc["@mozilla.org/intl/ospreferences;1"].getService(
+  Ci.mozIOSPreferences
+);
+
+const localeService = Services.locale;
+
+/**
+ * Make sure the locale service can be instantiated.
+ */
+
+add_test(function test_defaultLocale() {
+  const defaultLocale = localeService.defaultLocale;
+  Assert.ok(defaultLocale.length !== 0, "Default locale is not empty");
+  run_next_test();
+});
+
+add_test(function test_lastFallbackLocale() {
+  const lastFallbackLocale = localeService.lastFallbackLocale;
+  Assert.ok(lastFallbackLocale === "en-US", "Last fallback locale is en-US");
+  run_next_test();
+});
+
+add_test(function test_appLocalesAsLangTags() {
+  const appLocale = localeService.appLocaleAsLangTag;
+  Assert.ok(appLocale != "", "appLocale is non-empty");
+
+  const appLocales = localeService.appLocalesAsLangTags;
+  Assert.ok(Array.isArray(appLocales), "appLocales returns an array");
+
+  Assert.ok(
+    appLocale == appLocales[0],
+    "appLocale matches first entry in appLocales"
+  );
+
+  const enUSLocales = appLocales.filter(loc => loc === "en-US");
+  Assert.ok(enUSLocales.length == 1, "en-US is present exactly one time");
+
+  run_next_test();
+});
+
+const PREF_REQUESTED_LOCALES = "intl.locale.requested";
+const REQ_LOC_CHANGE_EVENT = "intl:requested-locales-changed";
+
+add_test(function test_requestedLocales() {
+  const requestedLocales = localeService.requestedLocales;
+  Assert.ok(
+    Array.isArray(requestedLocales),
+    "requestedLocales returns an array"
+  );
+
+  run_next_test();
+});
+
+/**
+ * In this test we verify that after we set an observer on the LocaleService
+ * event for requested locales change, it will be fired when the
+ * pref for matchOS is set to true.
+ *
+ * Then, we test that when the matchOS is set to true, we will retrieve
+ * OS locale from requestedLocales.
+ */
+add_test(function test_requestedLocales_matchOS() {
+  do_test_pending();
+
+  Services.prefs.setCharPref(PREF_REQUESTED_LOCALES, "ar-IR");
+
+  const observer = {
+    observe(aSubject, aTopic, aData) {
+      switch (aTopic) {
+        case REQ_LOC_CHANGE_EVENT:
+          const reqLocs = localeService.requestedLocales;
+          Assert.ok(reqLocs[0] === osPrefs.systemLocale);
+          Services.obs.removeObserver(observer, REQ_LOC_CHANGE_EVENT);
+          do_test_finished();
+      }
+    },
+  };
+
+  Services.obs.addObserver(observer, REQ_LOC_CHANGE_EVENT);
+  Services.prefs.setCharPref(PREF_REQUESTED_LOCALES, "");
+
+  run_next_test();
+});
+
+/**
+ * In this test we verify that after we set an observer on the LocaleService
+ * event for requested locales change, it will be fired when the
+ * pref for browser UI locale changes.
+ */
+add_test(function test_requestedLocales_onChange() {
+  do_test_pending();
+
+  Services.prefs.setCharPref(PREF_REQUESTED_LOCALES, "ar-IR");
+
+  const observer = {
+    observe(aSubject, aTopic, aData) {
+      switch (aTopic) {
+        case REQ_LOC_CHANGE_EVENT:
+          const reqLocs = localeService.requestedLocales;
+          Assert.ok(reqLocs[0] === "sr-RU");
+          Services.obs.removeObserver(observer, REQ_LOC_CHANGE_EVENT);
+          do_test_finished();
+      }
+    },
+  };
+
+  Services.obs.addObserver(observer, REQ_LOC_CHANGE_EVENT);
+  Services.prefs.setCharPref(PREF_REQUESTED_LOCALES, "sr-RU");
+
+  run_next_test();
+});
+
+add_test(function test_requestedLocale() {
+  Services.prefs.setCharPref(PREF_REQUESTED_LOCALES, "tlh");
+
+  let requestedLocale = localeService.requestedLocale;
+  Assert.ok(
+    requestedLocale === "tlh",
+    "requestedLocale returns the right value"
+  );
+
+  Services.prefs.clearUserPref(PREF_REQUESTED_LOCALES);
+
+  run_next_test();
+});
+
+add_test(function test_requestedLocales() {
+  localeService.requestedLocales = ["de-AT", "de-DE", "de-CH"];
+
+  let locales = localeService.requestedLocales;
+  Assert.ok(locales[0] === "de-AT");
+  Assert.ok(locales[1] === "de-DE");
+  Assert.ok(locales[2] === "de-CH");
+
+  run_next_test();
+});
+
+add_test(function test_isAppLocaleRTL() {
+  Assert.ok(typeof localeService.isAppLocaleRTL === "boolean");
+
+  run_next_test();
+});
+
+add_test(function test_isAppLocaleRTL_pseudo() {
+  let avLocales = localeService.availableLocales;
+  let reqLocales = localeService.requestedLocales;
+
+  localeService.availableLocales = ["en-US"];
+  localeService.requestedLocales = ["en-US"];
+  Services.prefs.setCharPref("intl.l10n.pseudo", "");
+
+  Assert.ok(localeService.isAppLocaleRTL === false);
+
+  Services.prefs.setCharPref("intl.l10n.pseudo", "bidi");
+  Assert.ok(localeService.isAppLocaleRTL === true);
+
+  Services.prefs.setCharPref("intl.l10n.pseudo", "accented");
+  Assert.ok(localeService.isAppLocaleRTL === false);
+
+  // Clean up
+  localeService.availableLocales = avLocales;
+  localeService.requestedLocales = reqLocales;
+  Services.prefs.clearUserPref("intl.l10n.pseudo");
+
+  run_next_test();
+});
+
+add_test(function test_packagedLocales() {
+  const locales = localeService.packagedLocales;
+  Assert.ok(locales.length !== 0, "Packaged locales are empty");
+  run_next_test();
+});
+
+add_test(function test_availableLocales() {
+  const avLocales = localeService.availableLocales;
+  localeService.availableLocales = ["und", "ar-IR"];
+
+  let locales = localeService.availableLocales;
+  Assert.ok(locales.length == 2);
+  Assert.ok(locales[0] === "und");
+  Assert.ok(locales[1] === "ar-IR");
+
+  localeService.availableLocales = avLocales;
+
+  run_next_test();
+});
+
+/**
+ * This test verifies that all values coming from the pref are sanitized.
+ */
+add_test(function test_requestedLocales_sanitize() {
+  Services.prefs.setStringPref(
+    PREF_REQUESTED_LOCALES,
+    "de,2,#$@#,pl,ąó,!a2,DE-at,,;"
+  );
+
+  let locales = localeService.requestedLocales;
+  Assert.equal(locales[0], "de");
+  Assert.equal(locales[1], "pl");
+  Assert.equal(locales[2], "de-AT");
+  Assert.equal(locales.length, 3);
+
+  Services.prefs.clearUserPref(PREF_REQUESTED_LOCALES);
+
+  run_next_test();
+});
+
+add_test(function test_handle_ja_JP_mac() {
+  const bkpAvLocales = localeService.availableLocales;
+
+  localeService.availableLocales = ["ja-JP-mac", "en-US"];
+
+  localeService.requestedLocales = ["ja-JP-mac"];
+
+  let reqLocales = localeService.requestedLocales;
+  Assert.equal(reqLocales[0], "ja-JP-macos");
+
+  let avLocales = localeService.availableLocales;
+  Assert.equal(avLocales[0], "ja-JP-macos");
+
+  let appLocales = localeService.appLocalesAsBCP47;
+  Assert.equal(appLocales[0], "ja-JP-macos");
+
+  let appLocalesAsLT = localeService.appLocalesAsLangTags;
+  Assert.equal(appLocalesAsLT[0], "ja-JP-mac");
+
+  Assert.equal(localeService.appLocaleAsLangTag, "ja-JP-mac");
+
+  localeService.availableLocales = bkpAvLocales;
+
+  run_next_test();
+});
+
+registerCleanupFunction(() => {
+  Services.prefs.clearUserPref(PREF_REQUESTED_LOCALES);
+});
diff --git a/intl/locale/tests/unit/test_localeService_negotiateLanguages.js b/intl/locale/tests/unit/test_localeService_negotiateLanguages.js
new file mode 100644
index 0000000000..f929bc3a43
--- /dev/null
+++ b/intl/locale/tests/unit/test_localeService_negotiateLanguages.js
@@ -0,0 +1,205 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+const localeService = Services.locale;
+
+const data = {
+  filtering: {
+    "exact match": [
+      [["en"], ["en"], ["en"]],
+      [["en-US"], ["en-US"], ["en-US"]],
+      [["en-Latn-US"], ["en-Latn-US"], ["en-Latn-US"]],
+      [["en-Latn-US-windows"], ["en-Latn-US-windows"], ["en-Latn-US-windows"]],
+      [["fr-FR"], ["de", "it", "fr-FR"], ["fr-FR"]],
+      [
+        ["fr", "pl", "de-DE"],
+        ["pl", "en-US", "de-DE"],
+        ["pl", "de-DE"],
+      ],
+    ],
+    "available as range": [
+      [["en-US"], ["en"], ["en"]],
+      [["en-Latn-US"], ["en-US"], ["en-US"]],
+      [["en-US-windows"], ["en-US"], ["en-US"]],
+      [
+        ["fr-CA", "de-DE"],
+        ["fr", "it", "de"],
+        ["fr", "de"],
+      ],
+      [["ja-JP-windows"], ["ja"], ["ja"]],
+      [
+        ["en-Latn-GB", "en-Latn-IN"],
+        ["en-IN", "en-GB"],
+        ["en-GB", "en-IN"],
+      ],
+    ],
+    "should match on likely subtag": [
+      [["en"], ["en-GB", "de", "en-US"], ["en-US", "en-GB"]],
+      [
+        ["en"],
+        ["en-Latn-GB", "de", "en-Latn-US"],
+        ["en-Latn-US", "en-Latn-GB"],
+      ],
+      [["fr"], ["fr-CA", "fr-FR"], ["fr-FR", "fr-CA"]],
+      [["az-IR"], ["az-Latn", "az-Arab"], ["az-Arab"]],
+      [["sr-RU"], ["sr-Cyrl", "sr-Latn"], ["sr-Latn"]],
+      [["sr"], ["sr-Latn", "sr-Cyrl"], ["sr-Cyrl"]],
+      [["zh-GB"], ["zh-Hans", "zh-Hant"], ["zh-Hant"]],
+      [["sr", "ru"], ["sr-Latn", "ru"], ["ru"]],
+      [["sr-RU"], ["sr-Latn-RO", "sr-Cyrl"], ["sr-Latn-RO"]],
+    ],
+    "should match likelySubtag region over other regions": [
+      [["en-CA"], ["en-ZA", "en-GB", "en-US"], ["en-US", "en-ZA", "en-GB"]],
+    ],
+    "should match cross-region": [
+      [["en"], ["en-US"], ["en-US"]],
+      [["en-US"], ["en-GB"], ["en-GB"]],
+      [["en-Latn-US"], ["en-Latn-GB"], ["en-Latn-GB"]],
+    ],
+    "should match cross-variant": [
+      [["en-US-linux"], ["en-US-windows"], ["en-US-windows"]],
+    ],
+    "should prioritize properly": [
+      // exact match first
+      [
+        ["en-US"],
+        ["en-US-windows", "en", "en-US"],
+        ["en-US", "en", "en-US-windows"],
+      ],
+      // available as range second
+      [["en-Latn-US"], ["en-GB", "en-US"], ["en-US", "en-GB"]],
+      // likely subtags third
+      [["en"], ["en-Cyrl-US", "en-Latn-US"], ["en-Latn-US"]],
+      // variant range fourth
+      [
+        ["en-US-macos"],
+        ["en-US-windows", "en-GB-macos"],
+        ["en-US-windows", "en-GB-macos"],
+      ],
+      // regional range fifth
+      [["en-US-macos"], ["en-GB-windows"], ["en-GB-windows"]],
+      [["en-US"], ["en-GB", "en"], ["en", "en-GB"]],
+      [
+        ["fr-CA-macos", "de-DE"],
+        ["de-DE", "fr-FR-windows"],
+        ["fr-FR-windows", "de-DE"],
+      ],
+    ],
+    "should handle default locale properly": [
+      [["fr"], ["de", "it"], []],
+      [["fr"], ["de", "it"], "en-US", ["en-US"]],
+      [["fr"], ["de", "en-US"], "en-US", ["en-US"]],
+      [
+        ["fr", "de-DE"],
+        ["de-DE", "fr-CA"],
+        "en-US",
+        ["fr-CA", "de-DE", "en-US"],
+      ],
+    ],
+    "should handle all matches on the 1st higher than any on the 2nd": [
+      [
+        ["fr-CA-macos", "de-DE"],
+        ["de-DE", "fr-FR-windows"],
+        ["fr-FR-windows", "de-DE"],
+      ],
+    ],
+    "should handle cases and underscores, returning the form given in the 'available' list":
+      [
+        [["fr_FR"], ["fr-FR"], ["fr-FR"]],
+        [["fr_fr"], ["fr-FR"], ["fr-FR"]],
+        [["fr_Fr"], ["fr-fR"], ["fr-fR"]],
+        [["fr_lAtN_fr"], ["fr-Latn-FR"], ["fr-Latn-FR"]],
+        [["fr_FR"], ["fr_FR"], ["fr_FR"]],
+        [["fr-FR"], ["fr_FR"], ["fr_FR"]],
+        [["fr_Cyrl_FR_macos"], ["fr_Cyrl_fr-macos"], ["fr_Cyrl_fr-macos"]],
+      ],
+    "should handle mozilla specific 3-letter variants": [
+      [
+        ["ja-JP-mac", "de-DE"],
+        ["ja-JP-mac", "de-DE"],
+        ["ja-JP-mac", "de-DE"],
+      ],
+    ],
+    "should not crash on invalid input": [
+      [["fą-FŻ"], ["ór_Fń"], []],
+      [["2"], ["ąóżł"], []],
+      [[[""]], ["fr-FR"], []],
+    ],
+    "should not match on invalid input": [[["en"], ["ťŮ"], []]],
+  },
+  matching: {
+    "should match only one per requested": [
+      [
+        ["fr", "en"],
+        ["en-US", "fr-FR", "en", "fr"],
+        null,
+        localeService.langNegStrategyMatching,
+        ["fr", "en"],
+      ],
+    ],
+  },
+  lookup: {
+    "should match only one": [
+      [
+        ["fr-FR", "en"],
+        ["en-US", "fr-FR", "en", "fr"],
+        "en-US",
+        localeService.langNegStrategyLookup,
+        ["fr-FR"],
+      ],
+    ],
+  },
+};
+
+function run_test() {
+  const nl = localeService.negotiateLanguages;
+
+  const json = JSON.stringify;
+  for (const strategy in data) {
+    for (const groupName in data[strategy]) {
+      const group = data[strategy][groupName];
+      for (const test of group) {
+        const requested = test[0];
+        const available = test[1];
+        const defaultLocale = test.length > 3 ? test[2] : undefined;
+        const strategyInner = test.length > 4 ? test[3] : undefined;
+        const supported = test[test.length - 1];
+
+        const result = nl(test[0], test[1], defaultLocale, strategyInner);
+        deepEqual(
+          result,
+          supported,
+          `\nExpected ${json(requested)} * ${json(available)} = ${json(
+            supported
+          )}.\n`
+        );
+      }
+    }
+  }
+
+  // Verify that we error out when requested or available is not an array.
+  for (const [req, avail] in [
+    [null, ["fr-FR"]],
+    [undefined, ["fr-FR"]],
+    [2, ["fr-FR"]],
+    ["fr-FR", ["fr-FR"]],
+    [["fr-FR"], null],
+    [["fr-FR"], undefined],
+    [["fr-FR"], 2],
+    [["fr-FR"], "fr-FR"],
+    [[null], []],
+    [[undefined], []],
+    [[undefined], [null]],
+    [[undefined], [undefined]],
+    [[null], [null]],
+    [[[]], [[2]]],
+  ]) {
+    Assert.throws(
+      () => {
+        nl(req, avail);
+      },
+      err => err.result == Cr.NS_ERROR_XPC_CANT_CONVERT_PRIMITIVE_TO_ARRAY
+    );
+  }
+}
diff --git a/intl/locale/tests/unit/test_osPreferences.js b/intl/locale/tests/unit/test_osPreferences.js
new file mode 100644
index 0000000000..6d1bb637ff
--- /dev/null
+++ b/intl/locale/tests/unit/test_osPreferences.js
@@ -0,0 +1,44 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+function run_test() {
+  const osprefs = Cc["@mozilla.org/intl/ospreferences;1"].getService(
+    Ci.mozIOSPreferences
+  );
+
+  const systemLocale = osprefs.systemLocale;
+  Assert.ok(systemLocale != "", "systemLocale is non-empty");
+
+  const systemLocales = osprefs.systemLocales;
+  Assert.ok(Array.isArray(systemLocales), "systemLocales returns an array");
+
+  Assert.ok(
+    systemLocale == systemLocales[0],
+    "systemLocale matches first entry in systemLocales"
+  );
+
+  const rgLocales = osprefs.regionalPrefsLocales;
+  Assert.ok(Array.isArray(rgLocales), "regionalPrefsLocales returns an array");
+
+  const getDateTimePatternTests = [
+    [osprefs.dateTimeFormatStyleNone, osprefs.dateTimeFormatStyleNone, ""],
+    [osprefs.dateTimeFormatStyleShort, osprefs.dateTimeFormatStyleNone, ""],
+    [osprefs.dateTimeFormatStyleNone, osprefs.dateTimeFormatStyleLong, "ar"],
+    [osprefs.dateTimeFormatStyleFull, osprefs.dateTimeFormatStyleMedium, "ru"],
+  ];
+
+  for (let i = 0; i < getDateTimePatternTests.length; i++) {
+    const test = getDateTimePatternTests[i];
+
+    const pattern = osprefs.getDateTimePattern(...test);
+    if (
+      test[0] !== osprefs.dateTimeFormatStyleNone &&
+      test[1] !== osprefs.dateTImeFormatStyleNone
+    ) {
+      Assert.greater(pattern.length, 0, "pattern is not empty.");
+    }
+  }
+
+  Assert.ok(1, "osprefs didn't crash");
+}
diff --git a/intl/locale/tests/unit/xpcshell.toml b/intl/locale/tests/unit/xpcshell.toml
new file mode 100644
index 0000000000..bbce8bd46e
--- /dev/null
+++ b/intl/locale/tests/unit/xpcshell.toml
@@ -0,0 +1,21 @@
+[DEFAULT]
+head =""
+support-files = [
+  "data/intl_on_workers_worker.js",
+  "data/chrome.manifest",
+]
+
+["test_bug22310.js"]
+skip-if = ["os != 'win' && os != 'mac'"]
+
+["test_intl_on_workers.js"]
+skip-if = ["os == 'android'"] # bug 1309447
+
+["test_langPackMatcher.js"]
+
+["test_localeService.js"]
+
+["test_localeService_negotiateLanguages.js"]
+
+["test_osPreferences.js"]
+skip-if = ["os == 'android'"] # bug 1344596
diff --git a/intl/locale/windows/OSPreferences_win.cpp b/intl/locale/windows/OSPreferences_win.cpp
new file mode 100644
index 0000000000..d790be7202
--- /dev/null
+++ b/intl/locale/windows/OSPreferences_win.cpp
@@ -0,0 +1,318 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "OSPreferences.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/intl/LocaleService.h"
+#include "nsReadableUtils.h"
+
+#include <windows.h>
+
+#ifndef __MINGW32__  // WinRT headers not yet supported by MinGW
+#  include <roapi.h>
+#  include <wrl.h>
+#  include <Windows.System.UserProfile.h>
+
+using namespace Microsoft::WRL;
+using namespace Microsoft::WRL::Wrappers;
+using namespace ABI::Windows::Foundation::Collections;
+using namespace ABI::Windows::System::UserProfile;
+#endif
+
+using namespace mozilla::intl;
+
+OSPreferences::OSPreferences() {}
+
+bool OSPreferences::ReadSystemLocales(nsTArray<nsCString>& aLocaleList) {
+  MOZ_ASSERT(aLocaleList.IsEmpty());
+
+#ifndef __MINGW32__
+  // Try to get language list from GlobalizationPreferences; if this fails,
+  // we'll fall back to GetUserPreferredUILanguages.
+  // Per MSDN, these APIs are not available prior to Win8.
+  ComPtr<IGlobalizationPreferencesStatics> globalizationPrefs;
+  ComPtr<IVectorView<HSTRING>> languages;
+  uint32_t count;
+  if (SUCCEEDED(RoGetActivationFactory(
+          HStringReference(
+              RuntimeClass_Windows_System_UserProfile_GlobalizationPreferences)
+              .Get(),
+          IID_PPV_ARGS(&globalizationPrefs))) &&
+      SUCCEEDED(globalizationPrefs->get_Languages(&languages)) &&
+      SUCCEEDED(languages->get_Size(&count))) {
+    for (uint32_t i = 0; i < count; ++i) {
+      HString lang;
+      if (SUCCEEDED(languages->GetAt(i, lang.GetAddressOf()))) {
+        unsigned int length;
+        const wchar_t* text = lang.GetRawBuffer(&length);
+        NS_LossyConvertUTF16toASCII loc(text, length);
+        if (CanonicalizeLanguageTag(loc)) {
+          if (!loc.Contains('-')) {
+            // DirectWrite font-name code doesn't like to be given a bare
+            // language code with no region subtag, but the
+            // GlobalizationPreferences API may give us one (e.g. "ja").
+            // So if there's no hyphen in the string at this point, we use
+            // AddLikelySubtags to get a suitable region code to
+            // go with it.
+            Locale locale;
+            auto result = LocaleParser::TryParse(loc, locale);
+            if (result.isOk() && locale.AddLikelySubtags().isOk() &&
+                locale.Region().Present()) {
+              loc.Append('-');
+              loc.Append(locale.Region().Span());
+            }
+          }
+          aLocaleList.AppendElement(loc);
+        }
+      }
+    }
+  }
+#endif
+
+  // Per MSDN, GetUserPreferredUILanguages is available from Vista onwards,
+  // so we can use it unconditionally (although it may not work well!)
+  if (aLocaleList.IsEmpty()) {
+    // Note that according to the questioner at
+    // https://stackoverflow.com/questions/52849233/getuserpreferreduilanguages-never-returns-more-than-two-languages,
+    // this may not always return the full list of languages we'd expect.
+    // We should always get at least the first-preference lang, though.
+    ULONG numLanguages = 0;
+    DWORD cchLanguagesBuffer = 0;
+    if (!GetUserPreferredUILanguages(MUI_LANGUAGE_NAME, &numLanguages, nullptr,
+                                     &cchLanguagesBuffer)) {
+      return false;
+    }
+
+    AutoTArray<WCHAR, 64> locBuffer;
+    locBuffer.SetCapacity(cchLanguagesBuffer);
+    if (!GetUserPreferredUILanguages(MUI_LANGUAGE_NAME, &numLanguages,
+                                     locBuffer.Elements(),
+                                     &cchLanguagesBuffer)) {
+      return false;
+    }
+
+    const WCHAR* start = locBuffer.Elements();
+    const WCHAR* bufEnd = start + cchLanguagesBuffer;
+    while (bufEnd - start > 1 && *start) {
+      const WCHAR* end = start + 1;
+      while (bufEnd - end > 1 && *end) {
+        end++;
+      }
+      NS_LossyConvertUTF16toASCII loc(start, end - start);
+      if (CanonicalizeLanguageTag(loc)) {
+        aLocaleList.AppendElement(loc);
+      }
+      start = end + 1;
+    }
+  }
+
+  return !aLocaleList.IsEmpty();
+}
+
+bool OSPreferences::ReadRegionalPrefsLocales(nsTArray<nsCString>& aLocaleList) {
+  MOZ_ASSERT(aLocaleList.IsEmpty());
+
+  WCHAR locale[LOCALE_NAME_MAX_LENGTH];
+  if (NS_WARN_IF(!LCIDToLocaleName(LOCALE_USER_DEFAULT, locale,
+                                   LOCALE_NAME_MAX_LENGTH, 0))) {
+    return false;
+  }
+
+  NS_LossyConvertUTF16toASCII loc(locale);
+
+  if (CanonicalizeLanguageTag(loc)) {
+    aLocaleList.AppendElement(loc);
+    return true;
+  }
+  return false;
+}
+
+static LCTYPE ToDateLCType(OSPreferences::DateTimeFormatStyle aFormatStyle) {
+  switch (aFormatStyle) {
+    case OSPreferences::DateTimeFormatStyle::None:
+      return LOCALE_SLONGDATE;
+    case OSPreferences::DateTimeFormatStyle::Short:
+      return LOCALE_SSHORTDATE;
+    case OSPreferences::DateTimeFormatStyle::Medium:
+      return LOCALE_SSHORTDATE;
+    case OSPreferences::DateTimeFormatStyle::Long:
+      return LOCALE_SLONGDATE;
+    case OSPreferences::DateTimeFormatStyle::Full:
+      return LOCALE_SLONGDATE;
+    case OSPreferences::DateTimeFormatStyle::Invalid:
+    default:
+      MOZ_ASSERT_UNREACHABLE("invalid date format");
+      return LOCALE_SLONGDATE;
+  }
+}
+
+static LCTYPE ToTimeLCType(OSPreferences::DateTimeFormatStyle aFormatStyle) {
+  switch (aFormatStyle) {
+    case OSPreferences::DateTimeFormatStyle::None:
+      return LOCALE_STIMEFORMAT;
+    case OSPreferences::DateTimeFormatStyle::Short:
+      return LOCALE_SSHORTTIME;
+    case OSPreferences::DateTimeFormatStyle::Medium:
+      return LOCALE_SSHORTTIME;
+    case OSPreferences::DateTimeFormatStyle::Long:
+      return LOCALE_STIMEFORMAT;
+    case OSPreferences::DateTimeFormatStyle::Full:
+      return LOCALE_STIMEFORMAT;
+    case OSPreferences::DateTimeFormatStyle::Invalid:
+    default:
+      MOZ_ASSERT_UNREACHABLE("invalid time format");
+      return LOCALE_STIMEFORMAT;
+  }
+}
+
+/**
+ * Windows API includes regional preferences from the user only
+ * if we pass empty locale string or if the locale string matches
+ * the current locale.
+ *
+ * Since Windows API only allows us to retrieve two options - short/long
+ * we map it to our four options as:
+ *
+ *   short  -> short
+ *   medium -> short
+ *   long   -> long
+ *   full   -> long
+ *
+ * In order to produce a single date/time format, we use CLDR pattern
+ * for combined date/time string, since Windows API does not provide an
+ * option for this.
+ */
+bool OSPreferences::ReadDateTimePattern(DateTimeFormatStyle aDateStyle,
+                                        DateTimeFormatStyle aTimeStyle,
+                                        const nsACString& aLocale,
+                                        nsACString& aRetVal) {
+  nsAutoString localeName;
+  CopyASCIItoUTF16(aLocale, localeName);
+
+  bool isDate = aDateStyle != DateTimeFormatStyle::None &&
+                aDateStyle != DateTimeFormatStyle::Invalid;
+  bool isTime = aTimeStyle != DateTimeFormatStyle::None &&
+                aTimeStyle != DateTimeFormatStyle::Invalid;
+
+  // If both date and time are wanted, we'll initially read them into a
+  // local string, and then insert them into the overall date+time pattern;
+  nsAutoString str;
+  if (isDate && isTime) {
+    if (!GetDateTimeConnectorPattern(aLocale, aRetVal)) {
+      NS_WARNING("failed to get date/time connector");
+      aRetVal.AssignLiteral("{1} {0}");
+    }
+  } else if (!isDate && !isTime) {
+    aRetVal.Truncate(0);
+    return true;
+  }
+
+  if (isDate) {
+    LCTYPE lcType = ToDateLCType(aDateStyle);
+    size_t len = GetLocaleInfoEx(
+        reinterpret_cast<const wchar_t*>(localeName.BeginReading()), lcType,
+        nullptr, 0);
+    if (len == 0) {
+      return false;
+    }
+
+    // We're doing it to ensure the terminator will fit when Windows writes the
+    // data to its output buffer. See bug 1358159 for details.
+    str.SetLength(len);
+    GetLocaleInfoEx(reinterpret_cast<const wchar_t*>(localeName.BeginReading()),
+                    lcType, (WCHAR*)str.BeginWriting(), len);
+    str.SetLength(len - 1);  // -1 because len counts the null terminator
+
+    // Windows uses "ddd" and "dddd" for abbreviated and full day names
+    // respectively,
+    //   https://msdn.microsoft.com/en-us/library/windows/desktop/dd317787(v=vs.85).aspx
+    // but in a CLDR/ICU-style pattern these should be "EEE" and "EEEE".
+    //   http://userguide.icu-project.org/formatparse/datetime
+    // So we fix that up here.
+    nsAString::const_iterator start, pos, end;
+    start = str.BeginReading(pos);
+    str.EndReading(end);
+    if (FindInReadable(u"dddd"_ns, pos, end)) {
+      str.ReplaceLiteral(pos - start, 4, u"EEEE");
+    } else {
+      pos = start;
+      if (FindInReadable(u"ddd"_ns, pos, end)) {
+        str.ReplaceLiteral(pos - start, 3, u"EEE");
+      }
+    }
+
+    // Also, Windows uses lowercase "g" or "gg" for era, but ICU wants uppercase
+    // "G" (it would interpret "g" as "modified Julian day"!). So fix that.
+    int32_t index = str.FindChar('g');
+    if (index >= 0) {
+      str.Replace(index, 1, 'G');
+      // If it was a double "gg", just drop the second one.
+      index++;
+      if (str.CharAt(index) == 'g') {
+        str.Cut(index, 1);
+      }
+    }
+
+    // If time was also requested, we need to substitute the date pattern from
+    // Windows into the date+time format that we have in aRetVal.
+    if (isTime) {
+      nsACString::const_iterator start, pos, end;
+      start = aRetVal.BeginReading(pos);
+      aRetVal.EndReading(end);
+      if (FindInReadable("{1}"_ns, pos, end)) {
+        aRetVal.Replace(pos - start, 3, NS_ConvertUTF16toUTF8(str));
+      }
+    } else {
+      aRetVal = NS_ConvertUTF16toUTF8(str);
+    }
+  }
+
+  if (isTime) {
+    LCTYPE lcType = ToTimeLCType(aTimeStyle);
+    size_t len = GetLocaleInfoEx(
+        reinterpret_cast<const wchar_t*>(localeName.BeginReading()), lcType,
+        nullptr, 0);
+    if (len == 0) {
+      return false;
+    }
+
+    // We're doing it to ensure the terminator will fit when Windows writes the
+    // data to its output buffer. See bug 1358159 for details.
+    str.SetLength(len);
+    GetLocaleInfoEx(reinterpret_cast<const wchar_t*>(localeName.BeginReading()),
+                    lcType, (WCHAR*)str.BeginWriting(), len);
+    str.SetLength(len - 1);
+
+    // Windows uses "t" or "tt" for a "time marker" (am/pm indicator),
+    //   https://msdn.microsoft.com/en-us/library/windows/desktop/dd318148(v=vs.85).aspx
+    // but in a CLDR/ICU-style pattern that should be "a".
+    //   http://userguide.icu-project.org/formatparse/datetime
+    // So we fix that up here.
+    int32_t index = str.FindChar('t');
+    if (index >= 0) {
+      str.Replace(index, 1, 'a');
+      index++;
+      if (str.CharAt(index) == 't') {
+        str.Cut(index, 1);
+      }
+    }
+
+    if (isDate) {
+      nsACString::const_iterator start, pos, end;
+      start = aRetVal.BeginReading(pos);
+      aRetVal.EndReading(end);
+      if (FindInReadable("{0}"_ns, pos, end)) {
+        aRetVal.Replace(pos - start, 3, NS_ConvertUTF16toUTF8(str));
+      }
+    } else {
+      aRetVal = NS_ConvertUTF16toUTF8(str);
+    }
+  }
+
+  return true;
+}
+
+void OSPreferences::RemoveObservers() {}
diff --git a/intl/locale/windows/moz.build b/intl/locale/windows/moz.build
new file mode 100644
index 0000000000..9ac234b354
--- /dev/null
+++ b/intl/locale/windows/moz.build
@@ -0,0 +1,13 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+SOURCES += ["OSPreferences_win.cpp"]
+
+FINAL_LIBRARY = "xul"
+
+LOCAL_INCLUDES += [
+    "..",
+]