Mismatch `LanguageIdentifier` type with other Fluent crates

[hongquan]

Other Fluent crates use the LanguageIdentifier from unic-langid, whereas fluent-langneg uses one from icu_locid, hence, the result of negotiate_languages cannot be used for other crates.

[zbraniecki]

Hi, yes, Fluent crates are not yet updated to switch to ICU4X. I'm working on it.

For now, for Fluent needs one should use older version of fluent-langneg, which is what fluent-rs does - https://github.com/projectfluent/fluent-rs/blob/main/Cargo.toml#L19