All language codes permissible for wikimedia projects (ie generally ISO 639-1 and -3) should be available for choosing in the sitelinks sections of Wikidata. When a code is entered of which there is no subdomain (of Wikipedia, Wikivoyage, etc.) yet, the target page should be searched on Incubator. (e.g. Wikipedia sitelinks, code liv, page name X => Search for incubator:Wp/liv/X and allow addition of the link if that page exists).
T37960: add config variable to hide incubator test wikipedia langlinks on wikipedia projects by default
T73685: [Task] Turn mediawiki.org into a Wikidata client
T206426: Storing multiple sitelinks to a multilingual wiki
list of wikis that have Wikimedia permanent duplicated pages (Q21286738)
Also proposed in Community-Wishlist-Survey-2016. Received 54 support votes, and ranked #18 out of 265 proposals. View full proposal with discussion and votes here.