I suggest C/C++ and Tcl/Tk. Together, they are quite powerful and Tk GUIs also work in Windows. I know Python tends to be the scripting language of choice here on /. but Tcl is quite widely used and if you are working in a field which has Tcl legacy systems (like EDA tools) then learning Tcl will give you a huge boost. Other than that, Perl for command line scripting might also be handy.