Language appears to be unique in humans and there are two different dedicated areas of the brain for the production and understanding of language. If one of those areas is damaged a person can’t produce language but they can understand it. If the other area is damaged they can’t understand but they can produce language even though it is ‘nonsense’. This is not limited to spoken language. People who sign a language have the exact same problems with signing as a spoken language person would have with the same damaged areas.
(The Great Courses – Understanding the Brain)