I think most people mean "polyglot programmer" when they do in fact know several languages in detail. I've got a number of languages for which I am not "a world-class expert" but I am certainly an "expert", able to structure significantly-sized systems with best-practices and a fairly detailed understanding of all relevant costs and benefits I'll get from it. Precisely because I've picked up so many languages, I also know that the hardest part of that is what you point out, that every single difference ends up affecting the best practices. With the exception of the very heavily bondage-and-discipline languages (Haskell, Rust, Ada, Idris), the mere act of learning syntax and most of the semantics of most languages is often a less-than-one day task.
True, but I've known people use it to mean something like language-agnostic technologist, with the idea that they have enough experience in one or two languages that they could pick up anything else at senior level.
> Precisely because I've picked up so many languages, I also know that the hardest part of that is what you point out
This is exactly it. The people I describe have a few similar languages under their belt, and simply extrapolate from this.
I wouldn't, for example, assume a experienced senior java dev could pick up Scala at the same level within a few months, despite the similarities; they simply lack experience with the new traits.
