We literally don't know. We don't understand how the brain stores concepts. It's not necessarily language: there are people that do not have an internal monologue, and yet they are still capable of higher level thinking.

Rilke: "There is a depth of thought untouched by words, and deeper still a depth of formless feeling untouched by thought."

I am one of those for what it’s worth. I struggle to put my thoughts into words, as it does not come naturally to me. When I think internally, I do not use language at all, a lot of of the time.

Drive my wife crazy as my answer to her questions are always really slow and considered. I have the first think what thought I want to convey, and then think “how do I translate this into words?”