I’m trying to create a group with a name in cyrillic script and the error message is "that “only letters, digits and underscore” is allowed. My guess is there’s an a-zA-Z regex, which doesn’t cover UTF-8. So please have in mind that latin script isn’t the only one in the world when doing validation.
Groupnames and usernames are ASCII, like Twitter. This is an intentional design decision.
The “full name” for the group or user can be 110% unicode.
Yes, Unicode support for usernames will be behind a site setting (default off). Group names use the same validation rules as usernames, so this change will essentially allow Unicode group names as well.