If a Chinese person who has been learning French for a year now wants to learn Italian, how do you go about teaching them Italian? You don't start from scratch and painfully relate Italian to Chinese: instead, you leverage their knowledge of French and teach Italian as, roughly, a variant of French: you highlight the delta between those two relatively close languages.
Similarly, after 6 weeks of teaching Racket I have now switched to OCAML: last year the way in which I did it was to teach OCAML from scratch, only going much more quickly than when I started Racket 6 weeks before. As I was doing it, I realized that it was largely redundant and boring. So this year, I simply show OCAML and corresponding Racket code side by side, so that the students see that many of the differences are simply syntactic details that can be translated automatically. That gives me more time to focus on the differences and try to figure out the reasons for those differences.
In hindsight, it is obvious.
I wonder if there are any web resources to serve that kind of requests: "I want to learn computer language X; I already know computer language Y".