Mozilla has a solution for you: See Change Default Mozilla Language.
A language pack is an extension that
changes the language of the user
interface in a Mozilla application
(Firefox, Thunderbird, SeaMonkey,
Downloading a language pack
To download a language pack:
- Go to: http://releases.mozilla.org/pub/mozilla.org/
- Go to the directory there for the Mozilla application you want to change—for example: thunderbird
- Go to the
- Go to the directory for the release you are using—for example: Thunderbird 126.96.36.199
- Go to the directory for your operating system—for example: win32
- Go to the
xpi directory there.
- Install the xpi for the language, or language and region, that you want. For example, the French language pack is: fr.xpi
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.
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
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