It's a bad idea. I've seen a couple of that kind of forums, where you have one global subject and sub-forum for that subject in few different languages. They all have been english native forums, but there was much more post in other languages, so i coudn't find an answer for anything.
Can'r remember well, but there was something like 10 post in english - nothing important, and thousands post in german, russian and some others.
Every time when i asked something on english side, i've been answerd whit a link to a german post...witch was not in english.
Multilingual website - yes! - it will help scirra to get more pople who understand what they read.
Multilingual forum - no!
I've asked Tom some time ago about integrating Google Translate into the forum - answer "I have considered it, and will look into it"