Currently, https://github.com/projectfluent/fluent.js/blob/8991a1760a83f1f9d8900e325ecc83dcb636337c/fluent-langneg/src/locale.ts only allow the primary language subtag, and extended language subtag is not implemented.

It also does not take into account of macrolanguages. For example, requesting zh when there is cmn-Hans available will not match cmn-Hans.