Probably the 1974 children’s TV series Land of the Lost, with the language Pakuni, created by UCLA linguist Victoria Fromkin.
I take the question as asking about languages developed beyond isolated words or language games, into at least a rudimentary conlang (constructed language). Wikipedia has a handy list of conlangs in fiction. Digging into their examples, the earliest ones in film and television are:
- 1974: Pakuni, Land of the Lost, created by Victoria Fromkin
- 1982: Vulcan, Star Trek (isolated words appeared from 1966; first developed into a rudimentary conlang in 1982 for The Wrath of Khan, by Marc Okrand)
- 1985: Klingon, Star Trek (isolated words appeared from 1979; developed into a basic conlang for The Search for Spock, again by Marc Okrand)
So Pakuni appears to be the first of these to be developed into a systematic, if rudimentary, conlang. Frath Wiki, a conlang wiki, supports the belief that this was the earliest:
Pakuni is of interest to the conlanging community primarily as a matter of historical fact. Later inventions, such as Klingon and Na'vi, are certainly more well known. […] Okrand [for Star Trek] and Frommer [for Avatar] and Peterson [Game of Thrones] have raised the bar immensely […] It is simply the case that Fromkin [creator of Pakuni] was showing them how to do it, way back in the 1970s.
There is a longer history of fictional conlangs in literature. Tolkien is widely credited for their first use, with the languages of Lord of the Rings (published 1954); this previous sff.se answer discusses and supports that claim. His languages of Middle-Earth were far more developed than the above early film/TV examples were when introduced.
(Note: Pakuni has already been named in @lucasbachmann’s answer, but I wanted to add some information and context to back up the claim that it’s the first.)