I took three years of Spanish and got an A every semester. Even when it was still fresh in my mind, I was nowhere near able to hold even a very simple conversation. And now just a few years later it's all totally gone from my brain.
My mother's native language is Spanish and she never taught me, which I resent her for. But I still find it incredible how shitty my public school education in Spanish was. We really should be teaching kids a second language from kindergarten up.
What do you mean it's "untested". You learned a language through comprehensible input and so did I. So did every single person on this website. It is the most tested method of language learning in human history.