We receive a lot of spam with chinese characters. However, they are all UTF-8 encoded, therefore the header check of the character set doesn't work. Maybe you could add a detection method which tries to guess the language based on the UTF-8 characters? E.g if the mail contains at least 1 chinese character, classify it as Chinese.
In GFI MailEssentials we have a Language Detection filter which is already able to do this mentioned functionality. It works by actually detecting the language and has support for both Traditional and Chinese languages. The functionality you mention as part of the Header checking is quite obsolete now and we suggest to use the new filter.
This was released as part of the 2014 version