Bug 1479377 - Provide fallbacks for switch language in Prefs message r=gandalf

MozReview-Commit-ID: G3oeSWl1Rtf

--HG--
extra : rebase_source : 7f2f208793a601b3ada1971cda827f50f3a8fda3

browser/components/preferences/in-content/main.js

@@ -235,10 +235,15 @@ if (AppConstants.MOZ_UPDATER) {
 // We store this in a global so tests can await it.
 var promiseLoadHandlersList;
 
-// Load the preferences string bundle for a given locale.
+// Load the preferences string bundle for a given locale with fallbacks.
 function getBundleForLocale(locale) {
+  let locales = Array.from(new Set([
+    locale,
+    ...Services.locale.getRequestedLocales(),
+    Services.locale.lastFallbackLocale,
+  ]));
   function generateContexts(resourceIds) {
-    return L10nRegistry.generateContexts([locale], resourceIds);
+    return L10nRegistry.generateContexts(locales, resourceIds);
   }
   return new Localization([
     "browser/preferences/preferences.ftl",