bbPress.org

use bbpress and WPML on this site. What we did was to setup 2 different forums, one for each language.
Then from the english menu we link to the english forum and from the spanish menu we link to the spanish forum.
It works fine with no changes made to bbpress.

Let me explain that you can see the same forum in different languages just by changing the URL. For example, you can see this same forum in english or spanish