Fix language statistics after messages have been moved to JSON

Statistics are now showing always no messages since languages.inc
does not manage to load the JSON messages.

Had to make LocalisationCache::readJSONFile() public so that it can
be used in  languages.inc; since all methods from LocalisationCache
return localisations merged with fallbacks, which is not what we
want here.

Fix for I918cfdc46c (0dd91d5).

Change-Id: Ib52287db618b9d072e847130070d165a3e7ae44b
Merge "Fix language statistics after messages have been moved to JSON"