I walked up to him and signed, "Hello. My name is Susan." He tried to copy that and did a sloppy rendition of "Hello, my name is Susan." Obviously he didn’t know what he was doing. It wasn’t language. And I was shocked. He looked Mayan and I thought, well, if he knew Mexican sign language, he wouldn’t try to copy. That’s not a normal thing to do, even if you don’t know the language.
Those born deaf and not taught sign language will invent their own sign language. There are no human cultures without language.
Spoken (or signed) language is natural for humans, and developed as a form of telepathy. I can get a thought that's in my head into your head over a great distance transmitted over noisy media in a short time with a few words. But the thought in my head isn't in the form of an extremely ambiguous language like English or ASL or whatever I happened to learn as a child. It's the other way around: the thoughts come first.
Check out The Language Instinct by Steven Pinker for a nice overview on the scientific understanding around this.
The book Adam's Tongue by Derek Bickerton chronicles one view of how language evolved in humans and in doing so, evolved humans along with it. It's pretty fascinating.
The book explains how our brains wouldn't really have the "thoughts" you speak of without having had language in the first place. I'm not sure precisely how this fits in to children who can't speak and who were never taught to sign, but my guess is it would introduce considerable mental stumbling blocks.
I walked up to him and signed, "Hello. My name is Susan." He tried to copy that and did a sloppy rendition of "Hello, my name is Susan." Obviously he didn’t know what he was doing. It wasn’t language. And I was shocked. He looked Mayan and I thought, well, if he knew Mexican sign language, he wouldn’t try to copy. That’s not a normal thing to do, even if you don’t know the language.