Missing documentation for BCP 47 support
I heard that ICU supports BCP 47 language tags. I actually see it work in many places - I pass them into APIs that are documented to take old-style locale strings or just language codes, and get properly localized behavior. But I can't prove to my code reviewer that this is actually supposed to work, because I can't find anything about BCP 47 language tags in the documentation, other than the functions uloc_forLanguageTag and uloc_toLanguageTag.
Some places I looked:
There should be some clear documentation stating where I can use BCP 47 language tags instead of old-style locale strings or language codes, and any gotchas to watch out for.