The bug

String argument type in Brigadier doesn't parse correctly if it's of string type SINGLE_WORD or QUOTABLE_PHRASE without quotes. It reads only a-z, 0-9 and some other basic ASCII characters.

Because of this, for example, I can't use Russian usernames (custom launcher) in commands without quoting, although minecraft works with them perfectly fine. In other Minecraft versions (1.12.2 or older) it works as it should.

Proposed fix: Read unquoted strings in StringReader class until first space, not until first 'not allowed' character.