All of these cases, if they are genuine, reinforce the view that language must be learned within a critical window of time during youth, or it will be too late. Experiments with song birds reach the same conclusion: If deprived of the sound of adult song at a particular age, young finches are never able to learn to sing their species' song. (The research, incidentally, was called the Kaspar Hauser experiments.)
These findings are prime evidence of how nature and nurture work together rather than in opposition to each other. The brain is innately designed to be open to experience, but only during a certain period. I like to call it "nature via nurture."