Wikipedia defines a constructed language, or conlang for short, as "a language whose phonology, grammar, and/or vocabulary have been consciously devised by an individual or group, instead of having evolved naturally." Some conlangs are devised for a culture in a work of fiction; others are intended as a worldwide second language.
SIL International maintains ISO 639-3, an international standard listing notable languages. A language for human communication can be registered if it has a literature. Languages not in ISO 639-3 are not eligible to be included in Translatewiki, the project to translate interface messages in MediaWiki.
Articles about a language in Wikipedia are subjected to a less rigorous notability standard. Under English Wikipedia's general notability guideline, any language that "has received significant coverage in reliable sources that are independent of the subject" is eligible for an article based on the information in those sources. Thus Wikipedia has an article about tlhIngan Hol, the language of space-faring Klingons in the TV series Star Trek, even if not articles written in that language.