Of course the internal monologue would be in sign language. Those would be the symbols a deaf person manipulates in order to think. Since sign language is kinetic, not auditory, the thinking would have more of a physical component. Sort of dancing inside their heads.
But think about people with hearing. We are also used to seeing the symbols. So part of our imagination is aural and part visual. A deaf person would have the visual component, too.
Think a sentence. I’m thinking “what does it feel like to think/” Try to “watch” it internally to see how it takes place. Do you hear it? See it? Feel it?
For me, it’s really none of those things. The words kind of leap up into some place from which I can type them. My ears are ringing at the moment, for some reason, which is unusual and annoying. But I think the process of typing the words is crucial for them to become fully formed. Just as I need to speak the words in aural communication in order to know what I’m thinking. I suspect the thoughts I can think are different depending on the mode of expression of those thoughts.
When I am thinking inside my head (as opposed to expressing thoughts outside), the thoughts come quickly and can be lost quickly. Many times I’ll be thinking along and talking or writing, and then the thoughts will go faster than I can get them out. When that happens, many thoughts are lost.
I’ll think of two points at once, and then I’ll remind myself that this is when I lose thoughts, so I try to keep a hold of them both, but inevitably, one comes out first and when I go looking for the other, it isn’t there any more. Sometimes if I replicate the chain of thought I can recover it. Other times, not.
I’m tempted to say that a thought doesn’t exist until expressed. But that’s not really true. There is definitely this line of patter—words—going on inside me. Where they come from is a mystery to me, and that’s a mystery I’d love to solve. I think they come from the mind that has no words. But if so, how do they get to the word mind? I’m imagining a soft, spongy floor surrounding a dark pool from which a geyser erupts from time to time, spewing thoughts into the air. The minions grab the words, and rush them into the main office, where “I” decode them and decide which ones to express to the outside world.