Junior is just a title, nominally it means you aren't experienced enough to work undirected. You could have amazing C++ language knowledge and still be a junior

Junior is just a title, nominally it means you aren't experienced enough to work undirected. You could have amazing C++ language knowledge and still be a junior. You could also have very little but be skilled with another language. Even at a senior level you can get jobs outside of your immediate language experience. The difference being that a senior is expected to have enough experience to quickly pickup the language without too much mentoring and not need help with much else work wise whilst a junior might need help in all sorts of domains. Almost everyone regardless of level requires some onboard taking on a new job. As an aside you should look at someone in aggregate rather than purely judging them by something dumb they did. If we all got judged so harshly on our silly mistakes or quirky knowledge gaps no one would be fit to program professionally. And yes one of the things you will do at a senior level is mentor others in language use.