I have worked in many, many languages in the past and I've always found it incredibly easy to switch, to the point where you're able to contribute right away and be efficient after a few hours.
I recently had to do some updates on a Kotlin project, having never used it (and not used Java in a few years either), and there was absolutely no barrier.
They don't struggle to write code, but they struggle to write idiomatic code.
An experienced programmer from a different language introduced to another will write lots of code that works, but in a style idiomatic to their favoured language, be that C, C++, rust, python, etc.
Every language has its quirks, and mastery is less about being able to write a for loop or any given algorithm in any given language, but more about knowing which things you should write, and which things you should be using the standard libraries for.
I've literally seen C# consultants waste time writing their favourite .NET LINQ methods into a javascript library, so they can .Select(), .Where(), etc, rather than using .filter, .map, etc.
Likewise I've seen people coming from C struggle to be as productive as they ought to be in C#, because they'd rather write a bunch of static methods with loops than get to grips with LINQ.
Fully understanding a runtime (or compiler behaviour for AOT languages) and what is or isn't in standard libraries isn't something that can be mastered in a few hours.
Just shellcheck the hell out of it until it passes all tests.