One problem in trying to teach a "conversion" type approach is that if you didn't really understand a thing in language A where it was maybe not too important, what do we tell you in language B where it's identical (or even mostly identical) but now crucial ?

If we teach you the important thing, language A people who did know wonder why we're wasting their time on a concept they knew, but if we do not teach it, anybody who got by in language A without this understanding is now screwed in B where it's very important.