localeFormatter

A LocaleFormatter for languageTag, a BCP 47 tag.

The tag is honoured as far as the platform's own data goes and falls back the way that platform falls back, so the formatter this returns may report a different SymbolLocaleFormatter.languageTag than was asked for. Asking for a tag no platform knows yields that platform's root or default data rather than an error, which is the degradation the specification asks of missing references.