I think another problem is that some folks have a ridiculous and unrealistic definition even of the words "master" or even "know." There are guys out there who think if you put know C++ on a resume, that you literally memorized Bjarne Stroustrup's entire C++ Programming Language book. That's silly and completely unrealistic. To know or to master something is to be able to accomplish great tasks/feats with it, not to be perfect with it and unable to learn anything else.
Someone who has mastered the English language doesn't mean someone who on a whim could spell every single word in the dictionary correctly, or who knows every single definition of every word in the dictionary by heart... For example, I'm sure you could have tripped up Shakespeare with some words...
This is why I think the "Oh you can't possibly know that many languages well" sentiment can be silly.
You can because you can know software engineering well and basic how-to-do-that-in questions can be answered by a simple Google or YouTube search. Many would say Shakespeare mastered English. Does this mean that Shakespeare could have written flawless legal documents at a law firm? Probably not.
This can also be seen in programming directly: I'd much rather hire a guy with 15 years of experience in back-end web development to do my back-end rather than a guy with 15 years of JavaScript experience necessarily. The reason why is because the back-end stuff is at some point basically all the same and accomplishing similar tasks, so even if the back-end guy with his experience worked mainly in another language, that may even be a good thing because he could bring more to the table from those other technologies.
**The only exception here is a teacher. Someone who is teaching a language should have more textbook-knowledge of the language itself to be able to explain every nook and cranny, because they are forming the foundation of the language in the minds of others.