Locale region, not language. Important distinction. For example I don’t believe Tencent would be used in Hong Kong (zh_HK).
So, if you set Traditional Chinese for language and United States for region (https://developer.apple.com/library/archive/documentation/Ma...), [[NSLocale currentLocale] countryCode] (which Apple compares to "CN" for this feature according to this comment https://news.ycombinator.com/item?id=21242628) will return US, and [[NSLocale currentLocale] localeIdentifier] will return zh_US.
localeIdentifier also returns some other interesting things, e.g. mine is "en_US@currency=EUR", because I customized Region formats.
(Can't argue with the second point.)