Foreign accent syndrome is a rare medical condition involving speech production… To the untrained ear, those with the syndrome sound as though they speak their native languages with a foreign accent; for example, an American native speaker of English might sound as though they speak with a south-eastern English accent, or a native British speaker might speak with a New York American accent.

…one that occurred in Norway in 1941 after a young woman, Astrid L., suffered a head injury from shrapnel during an air-raid. After apparently recovering from the injury she was left with what sounded like a strong German accent and was shunned by her fellow Norwegians.

-wikipedia, foreign accent syndrome.