When the document language is changed, update cursor language too

When the document language is changed, update cursor language too

Actually, the change is done only if the cursor language was the
document language already.

This fixes an trivial but annoying problem: create a new file (in
English), change language to your favourite language, then start to
write. Before this fix, the words come out in English, which does not
make sense.

Fixes bug #9586.