I think the general principle in any kind of coding project is to get the job done, preferably in the most efficient and least error-prone way. Being proficient in more languages makes it easier to choose the most appropriate language for the task. However, it isn't a good idea to spread yourself too thin. For me, I've chosen to focus on just two: C/C++ for number crunching, and Python for scripting (and if the situation calls for it, to even consider mixing Python and C/C++). Then I add to that some basic-to-intermediate-level bash scripting and HTML/CSS/Javascript, because these serve a somewhat different kind of purpose than the other two.