During a soccer match in Georgia, Reuben Nsemoh, a 16 yr old goalie, got kicked in the head, passed out, stopped breathing periodically and had to be airlifted to a hospital. He remained in a coma there for 3 days. Upon waking, he could only speak Spanish. He had been exposed to the language through friends but was never fluent in it. As he healed, he gradually lost his fluency and returned to speaking English. Problem is, he's not fully recovered and the family faces exorbitant hospital bills. If you'd like to help, check out their GoFundMe page.
Doctors have documented his strange state in other cases. I think it's the brain's way of compensating for temporary loss of function in other areas. For example, in 2012, an Australian man woke up from his week-long coma, speaking Mandarin fluently. He had taken Mandarin classes before the accident but have was never fluent. Here are more examples. What these brain-injury sufferers/polygots have in common is that, at one point during their lives, they had actually been exposed to that particular foreign language.
I've never heard of any one being able to speak a language to which their brains have never been exposed. Please post if you know of any cases.
Sources:
http://usatodayhss.com/2016/teen-soccer-goalie-wakes-up-from-coma-speaking-a-different-language/
http://www.dailymail.co.uk/news/article-2740708/Aussie-man-wakes-coma-car-crash-speaking-MANDARIN.html
