Steps to replicate the issue (include links if applicable):

• Add a language (e.g. en-us, nan-hani) to $wmgExtraLanguageNames in InitialiseSettings.php (warning: large file)
• Go to Special:PageLanguage (e.g. on Wikidata - only visible to admins and translation admins though)

What happens?:

The language is not included in the list of selectable languages.

What should have happened instead?:

The language should be included in the list of selectable languages.

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

This particularly affects Wikidata and Commons (which already have a lot of additional languages), but also the Multilingual Wikisource (which would like a lot of additional languages) - T202794