OK. I think the myth is busted.
According to
https://wiki.mobileread.com/wiki/MOBI there is a “locale” field in the mobi header record. By default if the metadata doesn't specify the locale, it is set to “0x0009”. But a .mobi created by Amazon or Mobipocket the field is set to “0x0404” correctly for Traditional Chinese (zh-tw), or “0x0804” for Simplified Chinese (zh-cn).
I have tried to make Calibre output the correct locale and it works! Try add a metadata <dc:language>zh-tw</dc:language> to the epub file then convert again.