"Javascript" though isn't Javascript. I mean it could be, but it's worthless in that context because what you actually need to be productive is familiarity with the browser DOM, current frameworks for frontend or backend, and basic project structures.
For Java it might be enterprise Java, it might be Android, in which case you need familiarity with those frameworks to be productive. For Swift it'll be iOS.
No one's "diving into" the codebase for an iOS app starting from "I mostly do backend web app development in Golang".
Are you an "expert" in all those things? I doubt it. In fact the worst possible thing you can generally write anywhere is that you're an expert in C++ which at this point is oh-so-many subdialects.