There are brain injuries that can cause one to lose a language so only one remains (a girl starting to study German lost her native tongue in a coma and could only speak the German she'd learned: http://news.discovery.com/human/coma...rl-german.html) and there is "Foreign Accent Syndrome", another brain anomoly: http://www.healthmango.com/healthgen...notable-cases/ but no one has ever spoken a known foreign language they did not ever come in contact with/study.

"Speaking in tongues" is a religious experience and is not a "language" and requires someone of the same religion to "interpret", etc. but is not a mental illness.