Fixed. Sorry about that.
If you want a workaround in the meantime, you can use something like this:
LANGUAGE ~Chinese~ ~schinese~
~test/english/prompts.tra~
~test/schinese/prompts-%WEIDU_OS%.tra~
This way the component names will be in Chinese while installing, but in English when written to the log file. Here English is used as the default language in situations when the Chinese tra file cannot be loaded (it's not intrinsically necessary to use English as default, but it is perhaps easiest). The problem is caused by the log being written inside a minimal variable scope, which erroneously did not include variables like %WEIDU_OS%.