Git-GUI does not allow you to set the language from within the application but tries to pick up the default language of your operating system.

If the default language is not the desired language, or if it picks up the wrong language (see Issue #255), the language can be changed by setting an environment variable: LANG="en_GB" (or whatever the desired language is).

To set the LANG environment variable globally on Windows (either for the current user, or for the entire system), see this guide.

If you don't want to set the environment variable globally, you can create a script or batch file. Alternatively, you can set the variable in the shell. For example, on Windows, open a Command Prompt (gitk.cmd doesn't work in the git-bash) and type:

C:\YOUR_LOCAL_REPO\> set LANG="en_GB"
C:\YOUR_LOCAL_REPO\> gitk.cmd

From gitk you can then select File->start git gui to start git-gui. As an alternative, or if you don't have a repo yet, you can use (please change the paths as necessary):

C:\YOUR_LOCAL_REPO\> set LANG="en_GB"
C:\YOUR_LOCAL_REPO\> "C:\Program Files (x86)\Git\bin\wish.exe" "C:\Program Files (x86)\Git\libexec\git-core\git-gui"

Be aware that some non-English localizations, such as German, even translate git commands such as 'pull' or 'fetch'.