Usually, you'll need two or more different languages to cover different purposes. The standard language for heavy duty physics simulations seems to be FORTRAN. Unfortunately, the old versions of FORTRAN are quite different from the more modern ones, and both are in use. But most of the analysis and plotting is done in some high level interactive language like IDL or MATLAB or Mathematica, since it's just so much faster to work with data. Python is rising in popularity.
If you are just starting out, then you should probably learn MATLAB. You won't need FORTRAN until you are working with huge simulations to be run on supercomputers.
Unfortunately, the languages that are often taught in University, like Java, C, Lisp, and assembly, are not used that much in physics. C is useful for everything, but it lacks the speed of FORTRAN or the conciseness of IDL.
It will also be useful to learn some sh.
Depending on your field, there might already be some dominant languages in the community. For fusion, IDL and FORTRAN are widely used.