For years i never understood why people said they think to themselves "some quote of what they were thinking*".
When someone asked multi-lingual people "What language do you think in" it occurred to me why.
For the same reason deaf people think in sign language. They literally have a voice in their head that is their own thoughts, spoken aloud on the inside?
I used to think things in ways that i don't have a way of representing. Could take an exam, get every answer right, and fail because they want' to know how i worked it out.
"Duhh, in my head"
"Then show your working on paper"
*doodle random lines*
"This means nothing, you fail".
If you grow up with no languages around you, you don't think in words. Just something i've noticed.