Why to restrict allowed usernames by different rules? For example why can’t user have “@123 qw” username? Is there any techical difficulties or it’s just about community rules?
Also is it ok to have national characters in the username? If I use “UTF-8” encoding for my website it should work just well in all browsers.
Username within a system is most of the time for the consumption of HUMANS therefore, from usability point of view it should be READABLE
And yes you can use your national characters in username and make sure you understand character encoding , storage and retrieval. You system/application should be ready to consume the selected encoding at every level e.g client-side, server-side and at database end and tools you use to manipulate with each tier e.g IDEs etc ..
So from my point of view you need some extra knowledge and efforts to handle such a system without killing Usability