I'm not sure for windows. I think you may need language support packages. Which language do you want to use? Some Gnutella programs have in-built language support. Since I don't know which one you're using or what you're wanting to do other than use another language on your computer, I can't really advise much.

Let me guess, you're using LW & it shows up in the wrong language is that right? If yes, then see Wrong language & how to change it back