bq. It was not without get regret, that I resolved on leaving the colony, because I had believed that my knowledge of the language and habits of the natives, acquired during my sojourning amongst them, might have led to my being employed by the local authorities during the rest of my life; but, when I reflected on the suspicion with which I was viewed by the most influential white men, and on the probable doubt the natives would entertain in my sincerity after having left them. I thought it best to retire to Van Diemen’s Land.

In 1802, when he was 20, William Buckley was sentenced to imprisonment for stealing a bolt of cloth. He escaped from imprisonment in Australia in 1803, and failing to find Sydney, he spent the next 33 years among the aborigines.

bq. In 1835 William Buckley appeared at the camp site of John Batman’s Port Phillip Association with a party of aboriginals who had told him about the sighting of a ship at Indented Heads.
He sat down. Waves of emotion swept over him. He felt weak and overwhelmed. How would he be received? He sweated with fear … He had his spears and hunting gear and wore possum skin clothing.
He had great difficulty being understood for he had forgotten his English language. Buckley could not understand their spoken English. He was offered bread. The word cleared a cloud from his brain. He understood further words. They tumbled over and over in his head as his native tongue came flooding back to him … The white men fed him and treated him with kindness. He showed them W.B. tattooed on his arm, and told them his story … [many inverted commas removed]

(from Australian Museums and Galleries Online by way of plep).
Here’s one of the pages, but the site is slow to load. Good contemporary illustrations. But it sounds as if Buckley had lost his A language…