I was hired into my current role where a lot of code is in C# never having written any C# in my life. That was about six weeks ago. The interviewers correctly assumed that since I have written numerous other semi-colon languages for 20+ years and know what I'm doing it won't be a problem. I use some modern C# idioms (because to me no idiom is more or less "modern" as I learned them all in the last six weeks) more than my colleagues do, and I think I'm perhaps squinting at the async feature wrong, but it's basically fine at this point.

I would not have taken a C++ role. But that's because I don't like C++. Could I learn C++? Presumably, I assume it'd take quite a bit longer than C# but I don't want to.

A lot of programming skill that's transferable from one role to another is also transferable from one general purpose language to another. Almost everything about programming generally is transferable. All the practical implications of architecture is transferable. All the theory is transferable. Data structures are transferable (albeit if you've got very narrow experience you may not be aware of just what's possible). A lot of high level APIs are transferable.

On the other hand corporate knowledge isn't. So, the fresh graduate hire who is an expert in your preferred language does not know that around here nobody above an Engineer Lead reads their Teams chat, and nobody above Group Manager reads email. They don't know that tweaking the CSS requires a Design Team Lead authorisation because once somebody made the text the wrong colour and branding were furious - and they equally don't know that "Normal Minor" changes take six months to approve around here so they should mark small changes "Urgent Minor" instead and then pester a Senior Engineer to OK them, but never pester a Senior Engineer if it needs a Config change because that is an ops problem and... you are going to need to hire internally if you actually need them to "hit the ground running".