1 files changed, 90 insertions, 0 deletions
diff --git a/toolkit/components/translations/tests/browser/browser_translations_actor_empty_langs.js b/toolkit/components/translations/tests/browser/browser_translations_actor_empty_langs.js
new file mode 100644
index 0000000000..132bb9a1f2
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+++ b/toolkit/components/translations/tests/browser/browser_translations_actor_empty_langs.js
@@ -0,0 +1,90 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Test some corner cases from Bug 1849815 where empty web languages were causing
+ * issues.
+ */
+add_task(async function test_detected_language() {
+  const { cleanup, tab } = await loadTestPage({
+    // This page will get its language changed by the test.
+    page: ENGLISH_PAGE_URL,
+    autoDownloadFromRemoteSettings: true,
+    // Empty out the accept languages.
+    languagePairs: [
+      // Spanish
+      { fromLang: "es", toLang: "en" },
+      { fromLang: "en", toLang: "es" },
+      // French
+      { fromLang: "fr", toLang: "en" },
+      { fromLang: "en", toLang: "fr" },
+    ],
+  });
+
+  async function getDetectedLanguagesFor(docLangTag) {
+    await ContentTask.spawn(
+      tab.linkedBrowser,
+      { docLangTag },
+      function changeLanguage({ docLangTag }) {
+        content.document.body.parentNode.setAttribute("lang", docLangTag);
+      }
+    );
+    // Clear out the cached values.
+    getTranslationsParent().languageState.detectedLanguages = null;
+    return getTranslationsParent().getDetectedLanguages(docLangTag);
+  }
+
+  {
+    const cleanupLocales = await mockLocales({
+      systemLocales: ["en"],
+      appLocales: ["en"],
+      webLanguages: [""],
+    });
+
+    Assert.deepEqual(
+      await getDetectedLanguagesFor("en"),
+      {
+        docLangTag: "en",
+        userLangTag: null,
+        isDocLangTagSupported: true,
+      },
+      "If the web languages are empty, do not offer a language matching the app locale."
+    );
+
+    await cleanupLocales();
+  }
+
+  {
+    const cleanupLocales = await mockLocales({
+      systemLocales: ["en", "es"],
+      appLocales: ["en"],
+      webLanguages: [""],
+    });
+
+    Assert.deepEqual(
+      await getDetectedLanguagesFor("en"),
+      {
+        docLangTag: "en",
+        userLangTag: null,
+        isDocLangTagSupported: true,
+      },
+      "When there are multiple system locales, the app locale is used."
+    );
+
+    Assert.deepEqual(
+      await getDetectedLanguagesFor("es"),
+      {
+        docLangTag: "es",
+        userLangTag: "en",
+        isDocLangTagSupported: true,
+      },
+      "When there are multiple system locales, the app locale is used."
+    );
+
+    await cleanupLocales();
+  }
+
+  return cleanup();
+});