Maintainer instructions: new language PRs

This is how the library's maintainers (just one person at the moment!) review pull requests that add new languages.

1. Make sure the pull request has both language definitions and tests; ask for things that are missing.
2. Make sure the new language code is correct in the list of ISO 639-1 codes. It should be two characters.
3. Run each of the tests through Google Translate to make sure nothing offensive is present in the submission.
4. Review the code.
5. Merge the pull request and thank the author. If it wasn't added to the credits list, you can ask them how they'd like to be credited.
6. Deploy a new version of the library, typically with a minor version bump.