ok, now it works, finally...
once the updates hit the repo they should get marked automatically, so after you installed the versions firefox-10.0-2 and your FF-language-pack firefox-<your language>10.0-4 then do this:
- restart Firefox,
- you should now be asked to allow the new language pack to be installed, click "I allow" and "Continue"
- restart Firefox and use the new language
If you do not follow the steps above you can still activate the new language pack, or disable any other language pack, by going selecting from the Firefox menubar -> Tools -> Add-ons -> Languages and then enable/disable whatever language you want.
hope this is it now,