UserLanguageLookup: Only return languages we support

Right now people can, via Babel, specify languages we don't
support at all and they'll appear in the interface (but wont
work as the underlying API doesn't support these codes).

Also region codes can be uppercase right now (eg. nds-NL) while
MediaWiki (and thus our API) expects all codes in lowercase.

Change-Id: I0461a2514de01e36d299b267c3230ba6f6acd4a6
Merge "UserLanguageLookup: Only return languages we support"