A language label
For this site only, there are more languages than those currently supported in Mediawiki; as the purpose of thios site is to help develop the translations, including in languages still not in Mediawiki, there's the Template:Languagename that provides local translations (notably when they are still not supported by the "#language:" parser function of Mediawiki which uses a very partial database with many missing data, or approximative data that have not evolved since years and were refined in relevant standards (notably in ISO 639, with most data vetted from The Linguist List and researches indexed in Glottolog, frequently many years before they are vetted in ISO 639, then incoporated to the IANA database for BCP47 and then vetted once a year in CLDR (which is also very incomplete except a for few major languages), and finally integrated in MediaWiki (where there are also various non standard uses kept and not corrected since many years, sometimes decennials).
The Template:Languagename however allows an immediate central change locally for use in portals, languages navboxes, without having to rewrite many pages. It can be discussed here. But note that Wikipedia is not the best source (even its English version) because various distinguised languages are merged. Other possible sources include Wikidata. Both these wikis still want to provide sources. This Translatewiki.net just attempts to cover the most recommended practices, backed by serious linguistic sources (academic) like Glottolog and The Linguist List, then BCP 47 (for variants and current best practices that preserves the upward compatiblity) then by ISO 639 (for the encoding and English-only names, but ISO 639 still does not have enough coverage, even for modern languages and provides no hints for their classification like the one made in Glottolog, and ISO 639 has not always been not very stable), then CLDR (for translations of language names to other non-English languages or for autonyms), then Wikipedia (English Wikipedia is not always the best source even if it has the widest coverage and it's a very instable source).
Sometimes some existing Wikipedia editions use the wrong codes, still kept for legacy reasons, but do not distinguish them as much as they could (or currently don't have the tools to allow useful distinctions). Here we won't focus on the existing decisions made locally in a single wiki, because Translatewiki.net is for all wikis, and projects (including those outside Wikimedia itself).