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.