But to answer the question... to get to a place where I can contribute meaningfully to a project? Depends on the language, but I was dropped into a PHP / Python shop once after doing nothing but Java and C++ for the preceding 10 years or so, and I was contributing both PHP and Python code in a few days. But if you were to drop me into a Haskell or CL shop, I have a feeling it would take a bit longer.
Not sure I can actually point to a single language that I have mastered (except maybe Visual Basic).
I enjoy several language now, but often can't remember syntax for any of them. I think that's the thing that fades as you learn multiple languages - syntax. I know when I should use a loop, I struggle to remember how to implement a loop.
But that's the comfort that comes with the experience. I can afford to forget individual moments in time, because I know I can spin back up very quickly, if need be. It just has its comically absurd moments at the very beginning of the spinning.
I've learned to stop quantifying my learning as the "gotta get it all" mentality, because "what do I need to learn to solve today's problems" has a pretty good streak going.
C# has built-in facilities to call C++ code in a managed sense. C# doesn't do pointers the same way C does, so all of the low level minutae to pull that off is a very powerful feature.
There are even some microcontrollers whose architecture doesnt support function pointers very well, ao just doing vanilla C stuff is essentially hardware limited.