[@_ck_ Participant 18 years, 3 months ago]

There used to be a plugin to do this:

https://bbpress.org/plugins/topic/usernames-i18n-fix/

but it won’t work under 0.9 – instead see this:

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