What part of the brain controls language understanding?

The answer is Wernicke's area.

Wernicke's area and Broca's area are two parts of the cerebral cortex responsible for speech. Wernicke's area controls language understanding while Broca's area controls language production. Wernicke's area is found in the left cerebral hemisphere. If Wernicke's area is damaged, the person will be diagnosed with fluent aphasia. This means that person will be able to fluently connect words (since Broca's area controls this part), but the person will not understand languages and the sentences will be meaningless.