The TIOBE Programming Community Index gives an indication on the most popular computer languages. No surprise the first three most popular languages are Java, C and C++. I was surprised to see a small decline on C#.
Another source of similar measurements is LanguageUsageIndicators (LUI). Both agree that the first three languages are Java, C, C++.
I believe the three most important languages are Java, C++, C and C#. The similarities of Java and C# are many so if you know one you can learn the other in a matter of days.
As for scripting languages add Perl and Python to your arsenal.
A programmer has to know several languages in order to be able to meet the requirements of most programming projects. In my opinion, the core skills that every programmer or software engineer must have are a deep understanding of discrete mathematics, 2d and 3d data structures, algorithm design, and design patterns. If someone knows the above it is very easy to express them in any programming language.
A way to keep your skills updated is to write small programs in three languages. That way you keep the language syntax alive even if you do not use all the languages at work.
A good online source for Mathematics, Computer Science, Physics and Engineering is the MITOPENCOURSEWARE MITOPENCOURSEWARE online. MIT online has a wealth of information and it is free.