From MozillaZine Knowledge Base
A language pack is an extension (add-on) that changes the language of the user interface in a Mozilla application (Firefox, Thunderbird, SeaMonkey, etc.).
For example, if you have an English version of Thunderbird, then the first button on Thunderbird's toolbar has the label "Get Mail" and the tooltip "Get new messages". But if you install the French language pack and switch the user interface to French, then the first button on Thunderbird's toolbar has the new label "Relever" and the tooltip "Relever les nouveaux messages".
When you install a language pack, you can assign a different user-interface language to each profile that you use. Alternatively, you can assign a different user-interface language to each command or icon that you use to start the Mozilla application.
Downloading a language pack
Note: To install a language pack in Thunderbird, do not click the link to the xpi file. Instead, open Thunderbird's Extensions or Add-ons window, then drag the link from your browser and drop it there. Alternatively, save the xpi file on your computer and install it in Thunderbird later.
If you are using the most recent shipping release, the quick path to the language pack .xpi file is:
Otherwise, to download a language pack for your release:
Assigning a language to a profile
To assign a user-interface language to a profile, set the preference general.useragent.locale to the name of the locale that you want to use. The name of the locale is usually the same as the name of the language pack that you installed (without the .xpi extension).
For example, to use French you can specify the locale as fr or fr-FR.
Note: If you often switch locales, there is a Locale-Switcher extension to make it easier.
Assigning a language in a command or icon
To assign a user-interface language to an icon that launches your Mozilla application, look in the icon's properties to find the command there.
To assign a user-interface language in a command, add the switch -uilocale followed by the name of the locale.
For example, to use French you can specify either: -uilocale fr or -uilocale fr-FR. On a Windows system the entire command might look something like:
"C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -uilocale fr
Note: as of 9/2010, the lists of language packs at AMO are fairly outdated.