I get the impression that people who say they have an internal monologue actually think out the words "I need to go to the store and buy milk" instead of just the concept of such.
To translate that into words, my mind goes through a process something like "Fucker need fuck. Box. Need box. What's in the box. Box fuck. Milk. Place of milk. Place of get stuff. Shop. Need to go to the shop and buy milk!" And I have to be careful not to vocalise anything except the last part.