Consider migrating from language-tags to langcodes

[koenedaele]

Currently we user our own language-tags (https://github.com/OnroerendErfgoed/language-tags) for handling language tags. There's another python implementation (https://github.com/rspeer/langcodes) that seems to be more complete and valid. This would allow us to stop maintaining language-tags and be more up to date with the entire ecosystem. Consider migrating to this library for version 2.