It's certainly possible that you'll miss some oddball idiomatic way of doing things in a new language (e.g. Python itertools or using C++ STL algorithms or something like that), but this is rarely a real problem. The job gets done correctly -- and in any case, we're all learning new ways of doing things. It's not as if you gain total prescience after N days on the job with Blub.
The point isn't that the generalist programmer will be 100% correct in all the details in new language, it's that they'll be able to quickly (and correctly) implement the important parts in whatever language you're using. Idioms tend to be the low-order bit of a solution anyway.
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.
