If there were someone who knew how to learn new languages, we would all know it. They would be uniquely and unquestionably skilled at producing new language speakers.

The thing is... There are people like that. They may not know how to teach that to others, but if you look at simultaneous translators, they don't just know another language. They know 4 or more fluently. And maybe pick up another one over summer holiday just for fun. (At least the ones I know did that)