My experience is that learning a new language -at a basic level- can be done quickly (like a couple of
weeks, not days).
But learning it at an advanced level, is a whole different matter.
I've been writing Swift since, literally, a few hours after it was announced, in 2014. I write Swift every day (like, seven days a week), and have been, since that first day.
I feel as if I have only had "mastery" of the language for the last couple of years. There's still plenty for me to learn, and they keep adding more stuff.
But the language isn't the big deal. The deal for me, is the fundamental SDK/API/Framework. That has been an ongoing project since 1986. As soon as I learn one API, they change it up (and sometimes, the entire OS).
During that time, I've written in 68K ASM, Pascal, Object Pascal, C, C+- (the weird THINK C variant), C++. ObjC, and Swift. I've worked with MacApp, OpenDoc(!), Think Libraries (Pascal and C), PowerPlant, and Cocoa (and its expression in iOS, WatchOS, AppKit, and TVOS).
I've always considered hiring folks to be an investment. As a manager, I always looked for folks that had the fundamentals down, and assumed that I could train them into what we need. Most of the work we did, was way beyond anything you'd find in textbooks. I was under no illusion that I could hire folks that would be able to start on it right away.
That was why my team was always one that I wanted to keep whole and cohesive, over many years. I can't even imagine today's 18-month engineer lifetime. I kept my engineers for many years, and that wasn't easy.