I'm sorry. No actual senior person who is senior at java or c# takes months of babysitting to switch to the other. The beauty of a senior person over someone who is junior and knows only js, is that the senior person should have used many languages over their careers. Picking up a new language and/or framework is what senior people should be doing best. Most of the factors that go into a proper maintainable solution are all language agnostic. Good design, DI, IoC, all of the 12 factor app suggestions are all language independent.
Even when I was back in college many years ago, only the first class taught a language. From that point forward the teacher of each class said we're using language X and suggested a book if you needed help learning.
I'm not the person you're replying to, but can you really argue that someone senior who doesn't know a language and its associated frameworks is equivalent to someone who does know those things or deserves to be paid the same?
Sure, it's true that many things about good design are language agnostic. On the other hand, frameworks and languages can actually limit or enable what you can do, and that lack of familiarity with them has the potential to lead to mistakes.
If everything else is equal, then obviously knowing the current language I need will push that person ahead. IME, the language(s) a senior person knows is the least important part whether they are a good hire though.
My reasoning for this is that we are always learning new languages and using new frameworks. Why would I let a better person go when the language is probably going to change, or worst case they pick it up in a couple weeks just by looking at the existing code base? One case where I would deviate a bit is if I was hiring for a functional programming position. In that case I would prefer experience with some functional language, but that is not much different than wanting OO experience for a Java/C# position.
If the more senior program is of the same calibre and lacks skill in a language, then 100% yes. The idea of discounting proper experience is a clear example of the Dunning Kruger effect.
Even when I was back in college many years ago, only the first class taught a language. From that point forward the teacher of each class said we're using language X and suggested a book if you needed help learning.