Considering you are knowledgeable at C, I would say your best bet is to learn C++. C++ is still widely used, and paves the way for other languages, such as Java, or Verilog if you're into hardware description languages.

Of course, if you are a very fast learner, I would suggest learning whatever languages employers are looking for. Really, to be safe, you could look at which languages are the most widely used, and learn them for future use.

I also should mention, all languages have their advantages and disadvantages. Depending on what you're trying to accomplish, which programming language could make a difference.