Chinese characters are not displayed in KMail's message window. I tried all possible encodings (from the display menu) and different fonts, in particular Chinese character fonts that display alright in every other KDE application.

Whe I answer the email, the cited text looks correct, so it's a problem of the message viewer only (and not the composer window).

You can replicate it by sending yourself a message with some Chinese characters from a webpage, for example.