What if a portion of your brain gets mutated into a different species?

Person A lost part of his brain due to some accident. The doctors decide to transplant a piece of brain from a healthy corpse. However, they realize that the point of the injury is too close to the brain stem; otherwise, there would not have been enough undamaged brain to cut through. By some misfortune, the doctors remove part of the stem along with a piece of the cerebral cortex.<br><br>Person B, on the other hand, was born with some genetic mutation that made their cerebral cortex grow into their cerebellum.<br><br>In either case, part of their brain gets taken over by a different animal. <br><br>What are the implications of this? How does Person B acquire language? Does it acquire language at all? Is there some sort of war between the two different types of animal in the brain? Is it conscious? How does the host's immune system respond to it?