Thrane It looks like a language extension for Danish hasn’t been published yet at https://github.com/flarum-lang/danish. You might need to discuss this matter with @rob006.
In the "Install a new extension" section of the Extension Manager, you generally just need to enter the part after composer require from the command you see on extension pages (e.g., flarum-lang/danish) into the vendor/package-name field and click "Proceed."
