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