More unified handling of domains

[MichaelChirico]

Now we have pt_BR, I think it's fine to just use pt.

We can also include logic to share columns of language_metadata.csv to the "parent" locale. Maybe split to a specific_language_metadata.csv to get full_name_eng/full_name_native for the regional locales? Since nplurals/plural are shared across all the regional locales.

That way if someone passes language=es_NI it doesn't fail, it just uses the es metadata and maybe shows a generic full_name_eng/full_name_native.