rebuildLocalisationCache: Implement --lang option

It would rebuild all language caches by default.
We were lacking an option to rebuild only one language, for example when
it get corrupted or the cache file for a language got removed.

The new --lang accepts a list of comma separated langs to rebuild the
cache for.  Example usage:

 php rebuildLocalisationCache.php --lang en,fr,de

Nonexistent languages are simply ignored. The script dies if it ends up
with no languages to rebuild.

Change-Id: I4c5e69abb76ffd8b88b595e3687edab3bb267ccc
Merge "rebuildLocalisationCache: Implement --lang option"