New feature request: Language

I'm using this cool library but i don't find any feature where the language can be modified just like country. I think it's great if you implement something like languagePreference and add/remove language programmatically. It'll be more awesome. Just my two cents, thanks for the lib!

No files, tests, or entry points are named in the issue. Start by locating the existing country selection and programmatic add/remove APIs, then determine the intended languagePreference behavior and how language data should be represented. Done means the feature's scope and API are defined and covered by tests.

索引モデルが issue の本文から書いたものです。