The phenomenon of xenoglossy (knowledge of a language one has never learned) is almost always associated with severe head injuries, recovery from comas or hypnotic "experiments."
Two types of xenoglossy are recognized: recitative and responsive. Recitative is the most common and people exhibiting it will utter words or phrases of an alien language without understanding their meaning.
Responsive is a much rarer form of xenoglossy: an individual is abruptly endowed with the capability of fluently speaking and conversing in a language completely (or virtually) previously unknown to them.
The latest case making world headlines is the rarer form of the phenomenon. According to doctors in Croatia, a 13-year old girl from Knin—a southern town—'mysteriously' fell into a coma. When she responded to questions put to her with fluent German! 
Her parents claim that while she had just started studying the German language at school, was reading German books and watching some German TV, she was far from being fluent in the language and just had a rudimentary grasp.
Dujomir Marasovic, the hospital director stated the staff were still in the process of determining what precipitated the coma and why it is the girl has forgotten her native language.
The director has called in other doctors to examine the girl—including German speaking doctors-in an attempt to unravel the oddity.
"You never know when recovering from such a trauma how the brain will react. Obviously we have some theories although at the moment we are limited in what we can say because we have to respect the privacy of the patient," Marasovic explained. 
Hospital staff psychologist, Dr. Mijo Milas observed, "In earlier times this would have been referred to as a miracle, we prefer to think that there must be a logical explanation—it's just that we haven't found it yet." 
Unfortunately, Dr. Milas may be looking for that explanation the rest of his life. What happened to the young Croation girl is nothing new. The phenomenon has been observed for hundreds of years.
Although anecdotal tales exist going back hundreds of years, one of the first well-documented occurrences of this phenomenon took place in 1862. During a demonstration of "mesmerizing" (hypnosis), a Prince Galtizin had a German woman who had no ability to speak any language other than her native tongue suddenly conversing in fluent French. Galtizin explained it as regression, claiming the woman had mentally reverted to a past life in France.
In the year 1977 an amazing xenoglossy case was documented. A Mr. Billy Mulligan, an Ohio State penitentiary inmate, was discovered by prison psychiatrists to have two distinct personalities. The first identity called himself "Abdul" and spoke flawless Arabic; the second, "Rugen" conversed in perfect Serbo-Croation. Yet according to the prison officials, Mulligan—who was a natural born citizen of the United States and had never left the country—was unaware of these other two personalities and had never studied either language!
Researcher and biologist, Lyall Watson recalls the case of a 10-year old Filipino boy who, when under a trance, spoke the Zulu language. He spoke it perfectly although he'd never even heard of it!
And back in 2007, a famous case involving Czech race car driver, Matej Kus, occurred after a near fatal accident on the racecourse. After regaining consciousness from his injuries, Kus-who previously had struggled to speak broken English-suddenly found himself speaking perfect English ... with a British accent!
His new found talent didn't last long, however, and he had to continue struggling with his English courses after a few days of glory. 
Some scientists have proposed that such cases might stem from a genetic legacy, while fringe science links it with the paranormal. 
Whatever the phenomenon proves to be, as Milas so succinctly sums it up: "There are references to cases where people who have been seriously ill and perhaps in a coma have woken up being able to speak other languages—sometimes even the Biblical languages such as that spoken in old Babylon or Egypt—at the moment though any speculation would remain just that—speculation—so it's better to continue tests until we actually know something."
 "Croatian teenager wakes from coma speaking fluent German," UK Telegraph, April 12, 2010
 "Croatian teenager wakes up from a coma speaking fluent German," Daily Mail, April 12, 2010
 "Croatian teenager wakes from coma speaking fluent German," UK Telegraph, ibbid.
 Czech speedway rider knocked out in crash wakes up speaking perfect English," Daily Mail
 "Xenoglossy: Evidence of Past Lives?" Leonardo Vintini, Epoch Times staff writer
Links to other cases of xenoglossy
Stroke gives woman British accent
Brain injury gives woman a foreign accent
My stroke left me with foreign accent
British charity worker recovers from car crash in Romania ... to speak with ROMANIAN accent
Man awoke from coma to find he had a foreign accent
Polish girl begins speaking Gaelic