i have an idea how to make its easy to implemet. i have also that same problem. i want to create a big site about linux, so it will include forums. so....
www.forum.pl.e-linux.info
www.forum.en.e-linux.info
www.forum.de.e-linux.info
at start i want to create these 3 forums. so, its no problem, but what if somebody wants to post on de and en? (know 2 languages?) creating user account on each one is not too good idea... so........
lets do it that way: in admin we creating categories. so let the category give subcategory. now:
category: polish
subcategory: distros
subcategory: mirrors
subcategory: linux as server
subcategory: linux as personal computer
etc.
that same for other languages. so, and when user visit the page forum automatically detects its language (or get it from profile settings) and switch to right category, but also user will be able to look to other languages.
how to make it better? lets copy punbb to 3 different directories. in config.php should be option
define("RESTRICT_TO_ONLY_ONE_CATEGORY", category_id);
and this restriction shows only one category, and also in search field will be only one category.
one database, multilanguage forum.
so... did you get the idea? understand me?