What happens to the language processing brain part if no language is ever learned?
Although this specific experiment wouldn't be done, similar experiments have been done in experiments in animal models where a sensory input is removed/altered. These experiments show that if you remove some sort of sensory input, the parts of the brain originally responding to that input will take on other tasks. There have been many studies that show this, but the canonical reference is often this one, where the part of the brain previously responding to sensory input to a finger instead responds to stimulation of another finger after the original finger is amputated:

Merzenich, M. M., Nelson, R. J., Stryker, M. P., Cynader, M. S., Schoppmann, A., &amp; Zook, J. M. (1984). Somatosensory cortical map changes following digit amputation in adult monkeys. Journal of comparative neurology, 224(4), 591-605.

There have also been studies of humans with sensory deficits like blindness or deafness that show similar reorganization, depending on the age the deficit is acquired. For examples, see:

Finney, E. M., Fine, I., &amp; Dobkins, K. R. (2001). Visual stimuli activate auditory cortex in the deaf. Nature neuroscience, 4(12), 1171-1173.

Kujala, T., Huotilainen, M., Sinkkonen, J., Ahonen, A. I., Alho, K., Hämälä, M. S., ... &amp; Salonen, O. (1995). Visual cortex activation in blind humans during sound discrimination. Neuroscience letters, 183(1), 143-146.

In summary, the result is that the neocortex is a very flexible, adaptable structure, and if inputs of a certain type are impoverished, that region of the cortex is used for other purposes. Essentially, neighboring functions creep in to that space.

Language processing in humans includes both auditory areas that respond to the sounds made by language, and premotor processing areas that seem to be important both for understanding the semantic content of language and for producing verbal output. None of these areas are truly language-unique, though, they are just regions that are active more than other areas when language-based tasks are occurring. In a person not exposed to a language, it's likely they would develop some rudimentary language-based skills anyways unless they were truly raised in some sort of sensory deprivation context - at this point an answer would get too far into speculation. Besides that, those areas would probably be re-purposed for other types of sensory processing and other types of motor planning.