1 files changed, 81 insertions, 0 deletions
diff --git a/toolkit/components/translations/tests/browser/browser_translations_actor_preferred_language.js b/toolkit/components/translations/tests/browser/browser_translations_actor_preferred_language.js
new file mode 100644
index 0000000000..0960e05b6e
--- /dev/null
+++ b/toolkit/components/translations/tests/browser/browser_translations_actor_preferred_language.js
@@ -0,0 +1,81 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+function waitForAppLocaleChanged() {
+  new Promise(resolve => {
+    function onChange() {
+      Services.obs.removeObserver(onChange, "intl:app-locales-changed");
+      resolve();
+    }
+    Services.obs.addObserver(onChange, "intl:app-locales-changed");
+  });
+}
+
+async function testWithLocales({
+  systemLocales,
+  appLocales,
+  webLanguages,
+  test,
+}) {
+  const cleanup = await mockLocales({
+    systemLocales,
+    appLocales,
+    webLanguages,
+  });
+  test();
+  return cleanup();
+}
+
+add_task(async function test_preferred_language() {
+  await testWithLocales({
+    systemLocales: ["en-US"],
+    appLocales: ["en-US"],
+    webLanguages: ["en-US"],
+    test() {
+      Assert.deepEqual(
+        TranslationsParent.getPreferredLanguages(),
+        ["en"],
+        "When all locales are English, only English is preferred."
+      );
+    },
+  });
+
+  await testWithLocales({
+    systemLocales: ["es-ES"],
+    appLocales: ["en-US"],
+    webLanguages: ["en-US"],
+    test() {
+      Assert.deepEqual(
+        TranslationsParent.getPreferredLanguages(),
+        ["en", "es"],
+        "When the operating system differs, it is added to the end of the preferred languages."
+      );
+    },
+  });
+
+  await testWithLocales({
+    systemLocales: ["zh-TW", "zh-CN", "de"],
+    appLocales: ["pt-BR", "pl"],
+    webLanguages: ["cs", "hu"],
+    test() {
+      Assert.deepEqual(
+        TranslationsParent.getPreferredLanguages(),
+        [
+          // webLanguages
+          "cs",
+          "hu",
+          // appLocales, notice that "en" is the last fallback.
+          "pt",
+          "pl",
+          "en",
+          // systemLocales
+          "zh",
+          "de",
+        ],
+        "Demonstrate an unrealistic but complicated locale situation."
+      );
+    },
+  });
+});