This will mean we may use 7000+ languages in Wikidata terms, monolingual texts and lexemes; Abstract Wikipedia will also benefit from it.
(For translating language names, see T231755: Local language name should be translatable in translatewiki)
(Not only those MediaWiki supported) See parent tasks for use cases. They should preferably be translatable, but there should be a way to prevent duplicates in translation.
Also we may adapt an existing MediaWiki extension, like CLDR.