Environment variables control the language settings from whichever terminal it is launched it will enhirit the LC_MESSAGES value for whichever language you wish, so set LC_MESSAGES=de and launch gimp in the same console window and vwalla (terrible french I know) and you have germen text. en is english if you need to know, also there is a complete list under the share\locale folder.

PS: one more reason to switch to Unix Alike Systems.