Make document_language non-mutable

The many occurances of the mutable keyword in LyX sources make it quite
difficult to understand what is really const and what not, and to debug
multithreading memory problems. In this particular case it is not needed
at all to have a mutable member, so make it non-mutable.
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.11.18-20.00