There are various lists defining language codes for Wikibase. Some values in these lists are not suitable for the termbox on Wikidata.org. See T44396 for the general problem.
The idea is to add a configuration variable that allows to disable such language codes. Deletion of existing values should still be possible.
* Sample: "no" isn't used on Wikidata, but is the domain name for no.wikipedia.org . Wikidata uses just "nb".
* Other codes for an initial version of the variable: "bat-smg", "bh", "fiu-vro", "roa-rup", "simple", "zh-classical, "zh-min-nan", "zh-yue", 'de-formal', 'es-formal', 'hu-formal', 'nl-informal',
Despite efforts to clean this up by bot, the numbers of are currently at 500,000 (June 2021, see T44396#7150919).
This doesn't touch their use for lexemes or monolingual strings. For the later, see [[ https://gerrit.wikimedia.org/g/mediawiki/extensions/Wikibase/+/77708f720c08e9e252bbe011b214bce6d6c777e6/lib/includes/WikibaseContentLanguages.php#142 | DifferenceContentLanguages ]] with DefaultMonolingualTextLanguages