Secret languages are often developed by siblings. They develop most frequently in identical twins. They typically fade away over time, and disappear by the age of three. However, special circumstances, such as social isolation, can lead to these languages persisting. This phenomenon is known as idioglossia, or cryptophasia.
The best known incidence of idioglossia was the language developed by the identical twins Grace and Virginia Kennedy, who referred to themselves as Poto and Cabengo.
The twins were born in Savannah. Georgia in 1970. They suffered severe convulsions shortly after birth. Although tests showed no brain damage, their father said a neurosurgeon told him it would take years to conclusively determine this.
The family moved to San Diego. Grace and Virginia did not socialize and rarely left the house. They spent most of their time either alone, or with their maternal grandmother, who spoke only German. They spoke very rapidly, and no one else could understand what they were saying. Neither spoke any English at the age of six. Private languages may have their own grammar and syntax, though that is very rare.
Phrases spoken by Virginia and Grace include:
La moa Poto - (here more poto)
Pinit, putatrahletungay (finish, potato salad hungry)
A documentary about the twins, "Poto and Cabengo", was released in 1980. Here is a brief clip.
A combination of speech therapy, psychotherapy, and the work of psycholinguists and speech pathologists helped to decipher the girls language. Over 100 hours of their playtime was taped and analyzed. It turned out that much of their language consisted of mispronounced and garbled English phrases,with a bit of German, that were spoken very rapidly. Most of their speech became somewhat intelligible after being slowed down. Their language contained 30 different words for potato.
Virginia and Grace managed to learn basic English.They were sent to separate schools to help facilitate their learning of English. Their IQ's tested as below average (around 80) but it's exceedingly difficult to tell what their real intelligence is.
According to Wikipedia, Ginny now works on an assembly line,and Grace works at a fast food restaurant.
Here's an interesting article on Grace and Virginia
The abstract of a scholarly article on idioglossia. It mentions that most of what is known about idioglossia is "folk knowledge" and that more scholarly research needs to be done.