Agreed overall, you will still be competent switching from one language to another, but intricacies and nuance matter a lot here. You may have enough knowledge to solve problems, but will you have enough knowledge to avoid creating new ones too? Like performance issues, or memory leaks, or other unwanted behavior? C++ is a great example here: someone that’s smart but inexperienced might just be dangerous enough to start writing classes with dumb pointers without overriding the copy constructors, and this is just a recipe for disaster.

I think it would take more than a few months to develop the kinds of experience that you need to be aware of these issues and avoid them. And while C++ is a very easy example to point out here, pretty much all languages have their share of footguns to be aware of, and it just takes time to learn them. A “deep knowledge” of a language is not just about being faster and more productive; it’s also about not creating more issues than the ones your solving.