For most people, the left half of the brain controls linguistic capabilities, but some people have their language centers in the right half  When a language center of the brain is damaged, for example by a stroke, linguistic capabilities are impaired in some way
Ttherefore, people who have suffered a serious stroke on the left side of the brain without suffering any such impairment must have their language centers in the right half
If there are language centers on the left side of the brain, any serious stroke affecting that side of the brain damages at least one of them.