Wrong question. In real life, no matter what you do, knowing just one language is not just enough; it would make you carrier vulnerable and usually even limit your understanding. More correct question would be: "what language is a must"? I would answer:
you cannot afford not knowing C#, no matter what you do, and knowing both C++ and C++/CLI is highly desirable.
But you should try to get familiar with many languages and their approach, and also with programming paradigms, and million other things, even if you don't use them in your everyday work.
As to languages, remember:
A language that doesn't affect the way you think about programming, is not worth knowing.
—SA