As I am wirting my articles in two languages here, I once in a while I need screenshots in a different language (in my case English, as I am running a Ubuntu in German). I don’t want log out to change the whole system. There is a simple command to run a single program in another language (as far, as you have installed the appropriate language package). To run Gimp (on an English system) in German, use:
$ LANGUAGE=de gimp