[nbouscal]

It really, really does not take "weeks or months" for a skilled programmer to pick up a new programming language. If you're using intrusive and opinionated frameworks where you have to learn a ton of conventions, that increases ramp-up time. If you're crossing paradigms, e.g. from Java to Haskell or from Python to C, that increases ramp-up time. But if you're just picking up a new programming language, say a switch from Python to Ruby? You can be productive in a week. I've done it before, more than once.

The difficulty isn't in finding people with specific experience, the difficulty is finding people with a solid enough understanding of essential concepts to be able to do good work regardless of the environment they're working in.