[_ck_]

There is a mini-plugin for i18n usersnames.

https://bbpress.org/forums/topic/diacritic-letters-i-username-like-goran#post-14205

But there are fundamental flaws in how bbPress uses usernames and unicode characters might interfere with proper operation.

I also don’t see any real attempt to sanitize in there, so there might be security risks.