New languages for monolingual text values on Wikidata can be defined in two places: in the wmgExtraLanguageNames setting in the Wikimedia production config repository, which makes them available for terms (labels/descriptions/aliases) as well, or in WikibaseContentLanguages::getDefaultMonolingualTextLanguages(), which doesn’t affect terms but is also effective on third party Wikibase installations.

Occasionally, language codes will migrate from only being available in monolingual text to being available in terms as well (e. g. T220118), which means adding them to wmgExtraLanguageNames in wikimedia-config. In the past, we’ve removed such codes from the default monolingual text languages afterwards (though often with some delay), since they’re now redundant in Wikidata. However, this means that third-party installations will lose support for such language codes even if they might (in theory) already have data using them, since the language codes are no longer in the default monolingual text languages and these installations don’t use the Wikimedia configuration where they’re now listed instead.

Should we do anything about this? I see several possible ways forward:

• Don’t change anything, continue to remove the redundant language codes. If third-party installations are unlikely to actually use those language codes, there’s no problem. If they do use them, they can still configure $wgExtraLanguageNames in their own settings.
• Fully duplicate wmgExtraLanguageNames in the default monolingual text languages, so that they’re supported on all installations. Very slightly bloats Wikibase, but it’s not really a big deal.
• Remove the hard-coded extra default monolingual text languages (i. e. this list) from the Wikibase source code repository and instead move them to the Wikimedia configuration repository as well, so they’re only effective on Wikidata and other Wikimedia installations, not on third-party installations. (Though I think the required configuration variable would have to be introduced first.) Once both lists are configured in the same place, we can remove a language code from the monolingual text languages every time we add it to wmgExtraLanguageNames, even in the same commit.