For some reason, by default, Gimp runs in french on my Windows 7 (English) desktop. I didn’t find any option to change language, in Gimp or GTK. So, to start Gimp in English, make a batch file containing something like:
set lang=en cd C:\Program Files (x86)\GIMP-2.0\bin gimp-2.6.exe
UPDATE: As it may also be useful for other apps peeking into environment variables and as a cleaner solution, we can set it globally or for the current user.
Control Panel\System and Security\System, Advanced system settings, Environment variables... name: lang, value: en