Fix bug #7741: incorrect locale when starting R from LyX?

Fix bug #7741: incorrect locale when starting R from LyX?

The goal here is to get rid of the old code that modified variables
LANGUAGE and LC_ALL, therefore creating the problems mentionned
in the ticket.

In the new system, there is no explicit "GUI" message handler, that
needs to be reset at each language change. Instead, getGuiMessages
calls getMessages with the correct parameter. This allows to simplify
greatly the code and to remove a lot of old cruft.