Now only the languages available for the selected file are displayed.
By default, LyX pre-selects the GUI language (and falls back to English
if this is not available). Once another language has been selected by
the user, this one is preferred before the GUI language (and the English
fallback).
This lists all respective files from user, build and system directory
in a structured way and thus allows for easier access to them.
Fixes: #2396, #6861
