The Old Man of Channelwood was a human that Atrus met in Channelwood. He had lived with the Tree-dwellers for most of his life and was the only last human amongst the people of that age, being revered because of it. By the time that Atrus finds him upon initially linking to the age, he's very nearly forgotten how to speak Atrus' language and is very eccentric. He eventually passes over his position to Atrus and commits suicide, jumping off of the high treetops to his death.
It is unknown whether the language Atrus referred to was D'ni language or English. Given that Atrus' journals were written in English, it is most likely the latter.