I see that what I said wasn't clear at all: I meant the specific kind of lead who will be setting everything up, ie. on a greenfield project. I think a lead on an established project falls into the camp of experienced people who can figure things out.

Another exception: contractors. In this case, you do want someone who is immediately productive in the language you're using because you do not reap any of the benefits of them ramping up like you do with permanent employees. For contractors, familiarity with the language and ecosystem is the more important part of their job; for employees, familiarity with the business is more important and the language stuff can be picked up.