Idk, I am someone, who has looked at many programming language, including all of those you mentioned. But a capable developer can be expected to learn a new language over the course of a few weeks if needed. I don't see how you could "run out of devs of language x", if you have capable devs on board. Especially, when those languages are all in the same programming language family/club.
Even the most capable developer that learns a new language in a few weeks will not be an expert in it. The difference in productivity and quality of the code will be huge. This is because in different languages things can be done very differently, it is not about the syntax as much as the best ways to do things.
