I'm sympathetic towards the idea but am not sure if the subforums would be used a lot? I've been part of the staff for some physics course with its own forum. There, we had a subforum where people could create threads for any language. My general observation was that threads for a few languages were made on the first day. People basically said hi, told where they were from, why they joined the course and after that, not much more. All serious discussion and questions for us about the lesson content eventually moved to the main forum. I've seen a similar pattern in other places.
Another slight disadvantage is that the forums get slightly less cohesive. Maybe cooperating with Shenmue sites dedicated to other languages is preferrable as others suggested?