One of the interesting footnotes of the political football that is the case of Sgt Bowe Bergdahl is that in five years the soldier lost much of his ability to speak in his native tongue. Some insight into such cases from the BBC Magazine:

“Some people have gone decades without speaking or hearing their first language but they retain the ability to speak it easily, says Dr. Monika Schmid, a linguistics professor at the University of Essex in the UK. But others begin losing fluency within a few years of not speaking it.

It’s rare to totally lose command of a first language, she says. Instead people have ‘language attrition’ – trouble recalling certain words or they use odd grammar structures. Age is a factor. Once past puberty, Dr Schmid says, your first language is stable and the effects of attrition can reverse themselves if you are re-immersed. But children as old as 10 don’t necessarily retain the language they were born into. In a study of French adoptees who left South Korea in childhood, when asked in their early 30s to identify Korean, they did no better than native French speakers with no exposure to the language.”