You can use non-latin characters in your password, but not in the username.
However, note that if you use such characters, your OS locale must match the one you set your password from. Otherwise, Winbox, as well as other applications, would convert the characters into potentially different code points. Alternatively, you'd need to write the equivalent code points into the other locale, which may be completely unrelated characters.
Also, I don't think you can type non-latin characters from a local console, so to prevent potentially locking yourself out, it's a good idea to keep around a separate account that only has latin characters in the password. You can make that account be ONLY usable from a local console to prevent abuse, but still - have it.
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