Ah yes, that's gonna be difficult... Would need some modifications to the forum software.
No it doesn't require modification at all. It just requires loading Arabic language files and enabling user-selectable languages support. It's a standard feature. I've done enough testing with it because I've had to do debugging for SMF 2.0 in RTL and I use Arabic as the test language.
(and I can tell you that debugging IE6 in Arabic when you don't read Arabic is highly amusing)
The whole software is set up to fully support either LTR or RTL languages. The only changes needed would be some additional RTL CSS for our custom themes, but I can sort that as all theming I do from now on is going to have RTL support anyway. However, I would prefer to not have to go back and debug RTL CSS for our current themes as we will be ditching them in the not too distant future when the site gets upgraded to 2.0. I would prefer to just do the 2.0 versions with RTL support.
The SMF default theme should be fine on RTL languages though even if it is a bit boring.
Anyway I'm fine with this sub-forum idea as long as we have some trusted members moderating it, or at least keeping an eye on it and reporting anything dodgey (with a translation, obviously).
Devious, treacherous, murderous, neanderthal, sub-human of the West.