LocalizationsThe packages in the repos are 'en-US' (US English) versions. To install any other translations:
* Download the appropriate language pack add-on .xpi file,
* Install it from within Firefox/Thunderbird/Seamonkey (as you would with any other add-on).
* Enable it by setting the preference general.useragent.locale to the name of the locale that you want to use. The name of the locale is the same as the name of the language pack that you installed.
* Restart the application for the setting to take effect.
Firefox language packs are at
http://releases.mozilla.org/pub/mozilla.org/firefox/releases/<version>/linux-i686/xpi/
Thunderbird language packs are at
http://releases.mozilla.org/pub/mozilla.org/thunderbird/releases/<version>/linux-i686/xpi/
Seamonkey language packs are at
http://releases.mozilla.org/pub/mozilla.org/seamonkey/releases/<version>/langpack/
For more info about installing language packs, see this
Mozilla knowledgebase article.