People assume that language acquisition is a form of sorcery.
Talent vs. Skill
There are really only two ways to acquire a language: One is to study, study, study, take classes and practice with peers; the other is to dive in and immerse yourself in a culture. People who have a ‘knack’ for languages almost always gain their languages via the latter process. They listen, they infer, and they quickly learn how to hack the sounds into what they need to say. It is a form of talent, certainly.
But whether you studied a language or learned how to converse in it by living in it, there must be a reason for you to use that language, or chances are very good you will forget the language. If you cease to speak a language on a regular basis, you will slowly lose the language – it’s almost inevitable for just about everyone.
How Languages Are Forgotten
The process of forgetting a language is gradual: At first there is a difficulty forming complex sentences, then a loss of vocabulary. This results in a hesitancy to speak the language at all, out of embarrassment. Thus a cycle is formed: You hesitate to use the language because you are forgetting it, and thus you forget it even faster due to lack of usage! If unchecked, eventually people even lose the ability to speak the language at all.
Curiously, the ability to comprehend the language is usually much more durable. Although subtleties may escape someone as their forgetting progresses, they often will be able to understand at least the basics of statements made in that language for years afterwards – sometimes for the rest of their lives.
There is no clear research on whether the forgetting of a language can be reversed easily. If you haven’t spoken Hindi in thirty years and suddenly return to the village in India where you learned it decades before, will the language return to you in a rush? Will at least your re-learning of the language be much faster than your initial acquisition? There are plenty of anecdotes on this subject, but no real research, so it remains a mystery.
Personally, I believe people do regain their lost languages very quickly – that the brain retains its knowledge in a hidden place, out of the way until needed.