Why so many languages based on ONE?
They all have different strengths and weaknesses, people wouldn't create new languages if there was no need to. Javascript is generally used for web stuff whereas C/C++ is not.

So basically -> if u know one ,you know 'em all??
If you understand the basic concepts of OOP then you've got a headstart in any languages that work with it. Getting a good grip on C++ will help you along no end in learning a language like Java and vv, as well as simpler languages like javascript.