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