@wonderland@plural.cafe pretty much the entire fediverse has agreed that the handles should be a restricted subset of ASCII (iirc letters, numbers, dashes, underscores; you can use upper and lower case but they are treated case insensitive). i think this is limited by what webfinger supports, but i haven't checked
technically dots are allowed, but i think most server softwares (at least misskey and mastodon do this) will only "hand out" user names with dots in them to system accounts. see e.g. accounts such as @plural.cafe@plural.cafe or @instance.actor@genau.qwertqefsday.eu
@wonderland@plural.cafe you can use umlauts in your display name later though of course...
@Johann150 yeah but i think it shoiuld be possible for everyone to have their own name as handle (if it is still free in the instance). not just in the display name. Quite a lot people do have names that are not made out of ascii characters.
Also it classifies languages and implies languages that are not using ascii as less worth. This is a problem in my opinion!
@wonderland@plural.cafe i did check and my best guess is that this restriction comes from the acct URI scheme defined in RFC 7565 § 6 and how Mastodon implemented it. Handles have to be part of an acct URI to be able to use webfinger, e.g. acct:wonderland@plural@cafe. That comes with the baggage of URIs and having to be ASCII.
From what I understand it would be technically within the RFC to have percent-encoded stuff in there. But the allowed characters in handles seems to be well agreed on so I don't want to know how many server softwares would vomit into their log files if you actually did that.
@wonderland it's not documented anywhere outside of github issues, but eugen made the decision to limit account names to "latin characters" (which ones specifically that is, i have no idea - ASCII probably) over concerns about impersonation and it being harder to mention others' account names