How does WMP and Real Player detect GB2312 in this situation? The computer codepage is Windows-1252 and the id3v2 tags says it is ISO-8859-1.

It's likely "guessing" the code page. Microsoft does this frequently, sometimes to disastrous effect (see: http://blogs.msdn.com/oldnewthing/archive/...3/24/95235.aspx)

I do not see how you can guess the encoding in tags. The tag is a very short text. "My software" by the way is the first file.

I have added a song with id3v2 tags encoded with GB2312. When read as GB2312 (and if You have a font supporting GB2312) you will see Chinese signs.