A masters in computer science program means taking about 10 three credit courses to get the degree. That means learning potentially 10 different languages. Which 10 would you choose? Which of those 10 are a must to learn, which would be merely advantageous to know?
Take at least one OO language (Java's fussy and bureaucratic, but its a pretty good example of the breed and is likely to be useful after you get your masters), at least one functional language (probably Haskell these days), at least one declarative language (Prolog or SQL), and don't just learn programming languages. You also need to learn about data, about data structures, about algorithms and their analysis, about parsing and compilation, and about concurrency; these are all independent of any programming language.
But computing is well served by not just learning about computing. If you have time, learn about math, stats and logic too, and learn how to communicate your ideas effectively; you'll never get far if you can't communicate with other people well.