I just tried to turn off the UTF8 bit in the Content-Type: text/html; charset=utf-8 header, but that totally messes up the Cyrillic forums. So I reversed this change as I value the forum posts and topic more then the generic forum language 
I noticed that the language pack files are encoded in ANSI while the Anpoop plugin translation by you was encoded in UTF8. The latter showed correctly, so you’re right that we would need UTF-8 language packs.
This whole language mess finally comes down to the non-UTF8-ness of punBB 1.2.x. The easiest option would be to upgrade to FluxBB 1.4 beta (which is the successor of the 1.2 series) which has native UTF8 support and some other neat stuff.