Hi Piet, that is very interesting about Vanilla Forums, thank you.
I have just looked at their pricing - $300/month minimum is going to be out of reach of my client, sadly. (This isn't a business project for him, but a "passion project" which is about ideas not money.)
The client needs WPML for the other parts of the site (he will be publishing in 2 languages), but I think the idea of having a single forum, where the language is decided by each topic, makes sense. It would be a lot easier to moderate, for starters...
thanks for your thoughts!