I wonder which is better :to start learning a new programming language or develop my skills in the C++ language ?
Both have advantages. What you should do depends on several things. One of these things is what is it that makes you face this dilemma. Do you have some specific goals in mind and you wonder if C++ is enough or there are better options (e.g. if you want to write a compiler, a functional language is by far a better choice), or are you just wondering which option is better for your evolution as a programmer?
In the first case, you'll first have to tell us what goals you have in mind.
In the second case, the answer depends on how much C++ you know. This -> "I started learning C++ since 7 months" doesn't tell us much. Walt, probably judging by personal experience, assumes that you're still a beginner, but this may not be the case. It would be better if you had said something like:
"I know conditionals, loops, arrays and functions, but I have a really hard time getting my head around pointers. I think I'll move to classes now and come back to that later."
Or something like:
"I have read and completely understood the tutorial on this site. I am familiar with using a big part of the standard library, and recently I started making my first step towards design patterns."
In the first sub-case, I think it's better to stick with C++. Moving to another language now would probably confuse you more.
In the second sub-case, learning another language wouldn't be a bad idea at all. But don't just learn any other language. Make sure that you also learn a different programming paradigm. Don't learn Java. Learn Haskell or Scala. And don't worry, the last 7 months won't go to waste. You'll never forget what you learned, and learning a functional language will also help you write better C++ code.