Doesn’t look pretty, but I’m not sure if I fully understand the scope of your request here, but then again, I’m one of those ignorant ASCII-natives who never had troubles with their character set (the only characters I’m struggling with are the german umlauts ä, ö, ü, etc., but you can easily work around that by using ae for ä, oe for ö, etc.).
Not sure if such a trick would also work for you or if there are latin equivalents of the usernames you’re trying to use here. I can image that having unicode username support would be beneficial for multiple countries with non-latin character sets, but obviously this is still not a standard procedure nowadays even in modern software solutions like Mattermost.
I’m pretty sure that someday the default will change, the more the libraries and infrastructure grow and get more mature, but as a sysadmin I can tell you like 200 top-of-the-notch software components widely used and enterprise messaging and infratructure solutions that are not even supporting extended ascii characters for things like usernames and e-mail addresses, so I’m afraid it’s still a long way until we can use such characters in all places without having to think about that.