I've seen "International forums" done on many other forums, but it's true it can pose a problem.
For example, on the
Team17 Forum you have one forum called International where people can just start threads in their own languages for others that understand it to reply. Lots of discussions go on in there. However, it's unmoderated, so there's also spam, although posts don't count.
On the
XcomUfo Forums, you have a category for multi-language forums. Each language which is demanded and has a moderator for it gets a forum. But in the end, it's a lot of mostly inactive forums :P
In the end, I think it's best if someone that can't write english just posts in their own language and whoever can understand it translates it for the rest.