I think the hardest part of a language's vocabulary (human language, not programming) to learn must be the technical language. And one specific area I thought of today that would be hard to find out (not necessarily to learn) would be the "management talk." You know, the fluffy language which paints a big picture rather than getting into nitty-gritty details.
Imagine you're a Chinese native speaker and you're learning English. Where would you learn that "down sizing" is a nicer term to use than "firing" or "making redundant" ? You aren't going to see it in a normal English language tutorial. Maybe they teach those terms on Business Language courses..
just a thought..