> A good developer will be able to deliver value regardless of the language they’ve used before.

This is true, but more in the long term. If I hire a brilliant and experienced .NET guy for my python team, sure he'll pick it up quickly and achieve some sort of productivity until he's up to par on PEP8 and idiomatic python. But I feel there are many cases where companies are looking for someone with experience in a specific language/framework in order to 'hit the ball running' aside from the usual getting-their-feet-wet with the codebase, learning the business, and other mythical man month issues.

[Edited for clarity.]