interesting. thinking about do the same in my forum.
great way to have 2 languages.
Except the promotion isn't instantaneous (so when you change language, you have to wait till the view is applied), it is really awesome.
Why? You can technically run 2 forums in 1 forum. Also your sidebar things will be treated like that (I use AndyB's, so events sidebar, recent posts, etc.).
That means, when someone is surfing in English, he/she only will see messages in English and Events in English, and the forum index will only show couple of nodes. So you don't have a large forum index page, which would be normally the case because it contains nodes for 2 languages.
And when someone is surfing in your primary language (Italian, Spanish, Polish, etc.), they will only see categories and messages (and sidebar stuff) only in their own language. That would be a bilingual forum. You could make that multi-lingual and make that 5 languages or whatever.
But the only thing is maybe indexing (waiting for an answer) and the promotion isn't instantaneous, so a bit confusing for new members.
But you can have 2 communities combined in 1 forum.