In 1978, French filmmaker and sometime Godard collaborator Jean-Pierre Gorin, then teaching at UC San Diego, trained his lens on eight year old identical twins Grace and Virginia Kennedy.
Poto and Cabengo, as the two girls called each other, spoke to each other in a private language they'd made up. Because their father feared they were developmentally disabled, the twins were neglected by their parents and kept from the outside world and their unique language flourished in that neglect.
Observing Poto and Cabengo in conversation is riveting, what one imagines watching a Henry Darger-directed version of 'Waiting for Godot' might be like.
'Poto and Cabengo' Digital Restoration from Criterion Films
(Jean-Pierre Gorin, 1979, USA, HDCam video, 76 min.) via Dangerous Minds
I hope someone will post the entire movie.