“Foreign accent syndrome (FAS) happens when you suddenly start to speak with a different accent. It’s most common after a head injury, stroke, or some other type of damage to the brain.  Although it’s extremely rare, it’s a real condition. Only about 100 people have been diagnosed with this condition since the first known case came to light in 1907. Some examples of FAS include an Australian woman who developed a French-sounding accent after a car accident. In 2018, an American woman in Arizona woke up one day with a mixture of Australian, British, and Irish accents after falling asleep the night before with a headache. It doesn’t just affect English speakers. FAS can happen to anyone and has been documented in cases and languages all over the world.”

healthline