Could you give a specific example as I am still thinking "random number generator" and want to balance it with your "standard idiom or library that everyone steeped in $lang knows "
Things like the Counter in python collections. Actually, the python collections library in general. You don't need that stuff to do the job, but it makes the code a lot clearer.
It probably took a couple of months full time in python before I had enough understanding of the std lib to know where to find everything (I sat down and read through a description of most of the functions in there).
There are still plenty of libraries that would have made my life easier if I knew about them. I'm sure there are plenty more.
Having said all that, that's just for example. When it comes to hiring I'd still pick a good experienced developer over a specific language expert for a full time position. For me it's not so much about the understanding of a specific language as it is the appetite for understanding computers / systems in general that makes truely good developers.
Ultimately the task is going to dictate the best type of hire. Short term contract, get a specific expert. Long term hire, find someone who actually enjoys developing software :)
I think there is a fine line between using a library and writin your own to understand better the domain
It's something to do with how critical the library functions are to you / your system. I would never write my own compression software, but I can see why people would just to learn about the trade offs.
Cheers