Well, I've been using Python for ten years, so I'm not the best example there. But, yeah, I'd be a better C# programmer than the vast majority of C# programmers with four years of experience, within two weeks. Of course my lack of knowledge of C# would get in my way all the time and slow me down, and I wouldn't be nearly as good as I would be after a few years of experience.
So why do I think I'd be a better C# programmer than most C# programmers in only two weeks? Because I'm a better programmer than most of them, and that's a more significant factor than language and library knowledge. A mediocre programmer can be mediocre in any language, and many of them continue to be mediocre for many years. Also, when you join an existing team with experience in a language, you can draw on their expertise.
There are a lot of programmers who are better than I am, thousands if not tens of thousands of them. I've had the privilege of collaborating with some of them. If someone had to choose between me and someone else at about my level, they'd be entirely justified in making the decision based on language knowledge. But the vast majority of experienced programmers are worse than I am --- much worse.
So why do I think I'd be a better C# programmer than most C# programmers in only two weeks? Because I'm a better programmer than most of them, and that's a more significant factor than language and library knowledge. A mediocre programmer can be mediocre in any language, and many of them continue to be mediocre for many years. Also, when you join an existing team with experience in a language, you can draw on their expertise.
There are a lot of programmers who are better than I am, thousands if not tens of thousands of them. I've had the privilege of collaborating with some of them. If someone had to choose between me and someone else at about my level, they'd be entirely justified in making the decision based on language knowledge. But the vast majority of experienced programmers are worse than I am --- much worse.