Language-data is hosted in https://github.com/wikimedia/language-data
It does have a package.json, but as far as I can see, it doesn't have any releases nor is it submitted to npm. It is not submitted to packagist.
Currently, I believe, composer dependencies are easier to add to MediaWiki core.
Steps to do
- Establish a release policy (who, when, semantic versioning)
- Make a release in GitHub
- Register language-data in npm
- Add composer.json and register in packagist