Ok, I see,
I just was remembering the "galleries profile" in which every user can administrate his subscribed threads and pictures. So I was thinking that this "profile" could be used to store a language information (= directory with languages files to use).
I this is not possible I would be really happy if a user could choose the language at every visit. Yes, this is annoying, but it's more than "oh, I cannot read anything here - bye!"
And I am sure that our hackers on photopost.dev will take this opportunity to hack (=integrate) this languages choice for the available forums - one by one

Even on vBulletin-germany.com there is a simple good hack for guests to choose their language at every visit. And other hackers took this to make more out of it for different things....
This "change language on click" would be simply a good start.
Thanks for your time,
Markus