This is a new package which provides types which enumerate human languages and language groups according to the ISO-369 standard. The standard has different parts depending on whether one is dealing with individual and macro languages or groups and families of languages, whether one uses two- or three-letter codes, and some historic quirks. See the project page and the API reference for further details.
This library is rather mundane, but I think it can help software dealing with multiple languages to make sure a language code is valid, and to make sure different language codes for the same language maps to the same language. I think the API is more or less in a final form unless there are usability issues which need to be addressed.