I don't disagree with the idea that becoming truly skilled in a language is more than a couple of weeks of work even for a great programmer, but more often than not you don't quite need that caliber of work from them right away.

Most job fills aren't going to be putting the new employee in charge of "greenfielding" the architecture of a brand new app, they'll be doing maintenance or build-out of an existing codebase, giving them plenty of time to ramp up on the fine details of a language while still being productive working with the existing code.