Posting in one language forum wouldn't post in another, but the forum structure would be the same (I would think descriptions should be the same, or blank, until they're updated with translations).
So, there could be a bar at the top with the language in it. Posts could be moved from one language forum to another.
Or, I guess you could make it optional, so when a certain language is needed a forum administrator can add it. That way there wouldn't be so many blank forums (although it wouldn't be so much like Wikipedia that way), and this would probably have to affect all parent and child forums (as well as, potentially, all cousin/sibling forums and such as are related).
Thanks for the suggestion. We will have foreign languages versions of Nabble at a certain point. But right now we don't have time for that yet. As for creating empty forums, there are a lot of empty forums in Google and Yahoo Groups which never go anywhere, so it may or may not work.