I voted "Never" because I almost never refer to language docs, by which I mean the language's syntax and semantics. If I use a language, I know how to make a for loop or how to define a class. If I start thinking about using some syntax which is obscure enough for me not to know it, I pause for a moment and decide to not use it, since if it's not obvious for me now, it's probably not going to be obvious for someone reading my code later on, either. One exception may be C++ which sometimes just won't do what I want it to without weird tricks. Or you can say I'm worse at C++ than at other languages I ever use. A completely different story are library docs (even if it's the standard library), which I use very often.