mnc is already in the list of monolingual text languages and is needed for lexemes too (see https://www.wikidata.org/wiki/Special:WhatLinksHere/Q33638?namespace=146)

There has been a MediaWiki translation exported from Translatewiki since February 2021, so it should eventually be added to Names.php (see T275740, T284043), but that does not seem likely to happen any time in the near future - it's been 1 year and 8 months already and I've made repeated attempts, unsuccessfully, over the past couple of months to get it moving.