You can select a different locale for an application from the command line by setting a couple of environment variables. First, make sure you have the appropriate language packs installed. If you can change your language (even if it requires restarting) it means you probably do. Then, on a terminal, do this (all on the same line):
LANG=fr_FR.UTF-8 LANGUAGE=fr_FR:fr firefox
This should start firefox with the french locale and everything within the app should display in french.
I know this is clunky to use and doesn't switch the entire UI over; you have to do it in an app-by-app basis. Hopefully it'll be helpful anyway.