2) Actually we do specifically intend to indicate that the Hong Kong accent is Cantonese. So how can we do this?
The example doesn’t really require Cantonese per se. It merely requires something that isn’t French.
If you want to indicate Cantonese specifically, then, prior to adoption of RFC 4646bis, you can use the tag “zh-yue”. This is a (grandfathered) registered tag. You can’t use the tag “zh-yue-HK”, at least you can’t use it today, since that one isn’t registered. After RFC 4646bis, depending on the outcome of the current discussion, the proper tag might be either “zh-yue-HK” or “yue-HK” (zh-yue would become a redundant or deprecated tag, but would still be valid).
Addison