SO is an English language site (the lingua franca of the IT world).
But there are lots of users names that are partially or wholly comprised with non-English characters, especially "non letters" (like symbols and non-letter glyphs). From a consistency aspect, why is that "OK", but posts that are not in English are "not OK"?
There is a practical aspect to this question too: Most people don't have easy access to foreign language keyboard input. When trying to respond to such a user, if the first character is a foreign character, it's tricky to get an auto-suggest for their user name after typing
@ in a comment. You have to use your mouse to copy-paste the user name in. If you happen to be using a mobile device, specifically an iPhone, this can be quite tricky.
This was not meant to ignite a cultural debate. It's just asking about consistency. It seems usernames are off limits for that.