Good programmers are good regardless of the language used. Poor programmers are poor regardless of the language used. Nobody is a "great Java programmer" and a "poor Python programmer." If they really are poor at Python, they probably were not great at Java! In addition, a really good programmer should be able to learn any new language easily. This is because a really good programmer understands and appreciates the concepts of quality programming, not just the syntactic and semantic idiosyncrasies of some programming language.
So the primary driver of language selection for a project should be appropriateness, not the surge of programmers who whine, "But all we know is Ruby." If some quit because the project selected a different language, the project is probably better off!
Reference:
Boehm, B., Software Engineering Economics, Englewood Cliffs, NJ: Prentice Hall, 1981.
https://www.youtube.com/watch?v=fkY7W6kCRY4
