I get what you are saying, but in some plugins that do this they automatically translate a users posts, which not be the right translation all the time. Some other plugins allow you to write a translation for each post, which I have no idea how you are going to do that for each users post in the forums.
Multilingual plugins are best for blog posts and pages in my opinion, while creating location specific forums or each language on a site in multisite might be better for forums.
If you do find a cool configuration to work with multilingual plugins then please share with the community.
