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I just noticed that my Firefox is in German. Then I noticed that my Thunderbird is in German, too.

Obviously, when I did a clean reinstall two weeks ago, I forgot to click through to the English version download. But after going through a whole system install, I just don't have the nerves to uninstall Firefox & Thunderbird, then do a new Firefox install with adding and tuning 15 addons, then Thunderbird reinstall including addons, setting up accounts and customizing tags. Is there a simple way to add an english language pack to my current installation or something?

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up vote 4 down vote accepted

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, etc.).

Downloading a language pack

To download a language pack:

  1. Go to:
  2. Go to the directory there for the Mozilla application you want to change—for example: thunderbird
  3. Go to the releases directory.
  4. Go to the directory for the release you are using—for example: Thunderbird
  5. Go to the directory for your operating system—for example: win32
  6. Go to the xpi directory there.
  7. 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 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
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Thank you, that saved me a lot of time and frustration – rumtscho Jul 15 '10 at 12:18
You're welcome. – Mehper C. Palavuzlar Jul 15 '10 at 12:19

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