Exactly, its not about languages as much, but how whole ecosystem works. Do you have some sort of VM? What libraries allow you to do? How does build works? What

Exactly, its not about languages as much, but how whole ecosystem works. Do you have some sort of VM? What libraries allow you to do? How does build works? What to expect from memory management? What kind of servers / clusters do you deploy to? And so on. Learning language to OK level isn't usually that hard. The rest of which makes an effective developer able to deliver useful products is another story.