Allow non-English wikis to rebuild l10n cache

If a wiki is configured to use the l10n cache and $wgLanguageCode is set
to anything other that 'en', all maintenance scripts will fail until the
l10n cache is build for the first time. This includes
maintenance/rebuildLocalisationCache.php, the maintenance script that
builds the l10n cache.

To break this dependency, have maintenance/rebuildLocalisationCache.php
force $wgLanguageCode = 'en'.

Change-Id: Ia93998b0f85fd122dd7b01eeeef698f595360945