“What part of the brain controls language?” The answer is that we do not know how, let alone exactly where, we decide what mix of words to use in order to convey a message. The systems studied by linguists are theoretical, formed from our conscious understandings. Any study by a linguist regarding the functioning of the human brain is, therefore, theoretical.
Language is typically controlled on the left side of the brain. Thinking of the brain in terms of sides, it is known in the medical sciences that the right side of the brain controls functions of the left side of the body, and the left side of the brain controls the functioning of the right side of the body. As speech and language are concerned, most people are right-hand dominant, and the language “center” of these people is on the left. Brain tumors and head injuries are a common occurrence. A person’s speech is often impacted in such events, particularly when the damage is on the left side of the brain.
There is an interesting medical case that happened in Norway during the 1940s. A woman, Astrid L., was in an accident, as the result of an air raid, and she suffered a serious injury on the left side of her brain. In the hospital, she regained consciousness but was paralyzed on her right side and unable to speak. In time, she recovered to the extent that she could walk on her own and speak fluently; her speech, however, was marked by the absence of “the natural Norwegian accent”.
Most theories are contrived by studying patients like Astrid L. Many of these studies compare the human brain to the electrical system in a building. For example, if a fuse blows and a classroom full of students goes dark, the system can be re-wired to provide electricity to where it is needed. Another interesting observation is that people who are right-handed have been able to learn to write with their left hand. This involves not just the muscles but using your brain as well.
While it is known that speech is controlled on the left side of the brain (for the majority of people), it is not known specifically where on the left side. Indeed, we should be careful not to jump to conclusions about something we have not designed and do not understand. The truth is that we do not know what is physically happening in the brain when it comes to an experience such as understanding a language.