Handle invalid language code gracefully in Language::fetchLanguageNames

Core MW doesn't have a problem, but translate extension explodes.
Seems safer to just use 'en' if passed an invalid language code.

Bug: 60629
Change-Id: I0ede3169bb49001a1515ae5b2b6d28477704f346
(cherry picked from commit 634905558b2b2a658d55ee98b3200d9bd0731fe4)