bbPress.org

If you are using bbPress 1.0, you can use a display name instead of your user login name, and the display name can be anything in the UTF8 character set.

There is also a workaround for bbPress 0.9 that might also work on 1.0 that removes all restrictions for the user login. It’s not been well tested to my knowledge but apparently works. I’ll try to find it for you.

Here it is, try this:

https://bbpress.org/forums/topic/accents-in-username#post-19037