I was intending to test out how to ping someone with a non-ASCII username - as part of the response to this post.
Since it specifically spoke about non-English languages, and frankly, it amused me, I tried to change the username of my alternate account from aibobot to ஐபோபோட். It's the same word transliterated to the Tamil script. We're the only language that uses it. Now I'm a little distracted and slightly confused as to why these are not accepted as letters.
Now, I can confirm that the letter ஐ is an actual letter. I wonder if the போ's are tripping it up (they are compound letters and have ligatures and other fancy typography), or maybe even the ட் is the problem. It tells me however that:
Display Name can only contain letters, digits, spaces, apostrophes or hyphens and must start with a letter or digit
Am I tripping up some less than obvious rule, or something put up due to abuse or spamming?
Full error message and example screenshot below.