Those born deaf and not taught sign language might not learn a language to think in at all. http://neuroanthropology.net/2010/07/21/life-without-languag...

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.