Interesting comment on Reddit about language, and in which language a person born deaf thinks.
So, to answer your question, I think in ASL (American Sign Language). I guess deaf people’s thinking process is little different from hearing people. When I think, it’s like I’m seeing myself signing from either my point of view or third person view and when I’m imagine a hearing person speaking, I imagine him/her actually signing instead of speaking because I can understand him/her that way. Also, we don’t always have a sign for every word in ASL. Sometimes when we want to think of a word that we don’t have a sign. We fingerspell it. It’s like imagining a letter by letter but only in hand shapes. I think in fingerspelling a lot when I’m reading an english sentence.