Bug 1410733 - Add an en-US locale as a hard fallback for LocaleService::RequestedLocales. r=jfkthame

MozReview-Commit-ID: LPS3PaeykEh

--HG--
extra : rebase_source : fd2ef3aaa10c809e93d4fe1bf9f962689c1bb9aa

intl/locale/LocaleService.cpp

@@ -292,6 +292,16 @@ LocaleService::GetRequestedLocales(nsTArray<nsCString>& aRetVal)
 {
   if (mRequestedLocales.IsEmpty()) {
     ReadRequestedLocales(mRequestedLocales);
+
+    // en-US is a LastResort locale. LastResort locale is a fallback locale
+    // for the requested locale chain. In the future we'll want to make the
+    // fallback chain differ per-locale. For now, it'll always fallback on en-US.
+    //
+    // Notice: This is not the same as DefaultLocale,
+    // which follows the default locale the build is in.
+    if (!mRequestedLocales.Contains("en-US")) {
+      mRequestedLocales.AppendElement("en-US");
+    }
   }
 
   aRetVal = mRequestedLocales;