Internally, Wikibase uses Language: fetchLanguageName which in turn uses Language: fetchLanguageNamesUncached ...which loads language names from various sources. Every time an item is loaded from Wikidata, it will place a fallback language behind the label if it is not available in the current language.
- In Danish the language "Dutch" is described on Wikipedia as https://da.wikipedia.org/wiki/Nederlandsk_(sprog) where hollandsk is an alias.
- Wikidata item https://www.wikidata.org/wiki/Q7411 has "nederlandsk" as label.
- The current name comes from CLDR which has "hollandsk" for nl.
The idea is to add "nederlandsk" to MediaWiki list.