Hi developer, currently Chinese characters are not displayed correctly due to the webpage isn't declaring a character encoding and my browser's automatic sets the encoding to Windows-1252, which doesn't support CJK characters at all.
IMO all webpages should declare using UTF-8 character encoding now.
Assuming that you are using UTF-8 encoding for Chinese characters in your /etc/passwd file, they should be displayed correctly by switching Usermin/Webmin to a UTF-8 character setup language.