Foreign accent syndrome (FAS) is speech disorder that causes a sudden change to speech so that a person speaks with a “foreign” accent. The condition is usually occurs after damage to the brain caused by a stroke or traumatic brain injury. Other reported causes include multiple sclerosis and conversion disorder. In some cases no clear cause can be identified.
Sixy cases of foreign accent syndrome have been reported between between 1941 and 2009 and include incidences from Japanese to Korean, British English to French, American-English to British English, and Spanish to Hungarian. (University of Texas, Wikipedia)