How to add a language that is not in the "Add a new Language" list?

I’m submitting a…

  • Bug report

Hello, I want to add a new language - English (Luxembourg) - to be supported in our page content. I went to Settings → Languages , I search for it in the dropdown list but couldn’t find it.
Is it possible to add support for this language code as well?

Current behavior

Can’t find & add the language.

Environment

App Name: Mine

  • Cloud version

Browser:

  • Chrome (desktop)

It is not a dropdown, just a autocompletion. You can basically add any country code you want. I really recommend to use the ISO syntax, [lanuage]-[COUNTRY] e.g. de-E. You don’t get a display name for it, but it is not really needed anyway.