Of course it is in the brain, the brain created and evolved the language as a very powerful tool. If intelligence was in the language then other animals would be as intelligent as us
LLMs learning from the same text and gaining human like capabilities shows just how much of intelligence is crystalized in culture. If it works without brains, then brains were not the essential ingredient.
Humans without culture would need 10,000 years or more to recover, and have to pay the same price as the first time around. Culture is smarter than us.
"If I have seen further, it is by standing on the shoulders of giants."
Other species have much more limited ability to transfer knowledge intergenerationally, and that is because the human brain's capability for symbolic language is much more advanced than other animals', who are not able to encode knowledge nearly as efficiently.
[0]: https://www.nobelprize.org/prizes/physics/1965/feynman/lectu...