http://doc.qt.io/qt-5/qlocale.html#QLocale-1

language is a lowercase, two-letter, ISO 639 language code (also some three-letter codes),

https://en.wikipedia.org/wiki/ISO_639

There are ISO 639-1, ISO 639-2, ISO 639-3, ISO 639-4, ISO 639-5 are still in use. I guess it should be ISO 639-1, that normally uses two-letter language code.

https://en.wikipedia.org/wiki/ISO_639-3
https://en.wikipedia.org/wiki/ISO_639_macrolanguage

At least for Chinese(zh in ISO 639-1, zho in ISO 639-3), I can't use cmn for Mandarin and yue for Yue (Cantonese) in Qt 5.9 code.