Think of it as as generic templating library/system. It has no knowledge of the output, or the differences. The benefit is that you write one "unified" say model, or some sort of input. And then write the translation to different languages (if you want, it can just be one), and have it generate for you automatically based on the rules you coded into the translation logic.

You write your own backends. These can be trivial to very comolex. See zproto or zproject for examples.

Thats really the key question here.

As I understand it, you do it by hand.