If you search around the 'net you'll find a number of cases in which a person's principle language has changed after a traumatic brain injury. Some people have even become fluent in a language they only recalled a pidgin vocabulary from before.

Perhaps more common, but still quite rare at about one person per year worldwide, is something called foreign accent syndrome. In which a head injury causes someone with one regional accent to speak the same language with a different accent afterward, often resembling the accent of another region. Examples have included from US to British, US to French, an Oregon woman developed an Australian one, etc.

Sensory changes, changes in personality, and other things can happen with stroke or accidental brain trauma, too.

One particularly interesting case is hinted at in this article, but if you're interested your favorite search engine should turn up many more cases.: https://www.thecut.com/2016/10/the-boy-who-came-out-of-a-com...