javascript/nodejs. I had been using Matlab ~15 yrs until I left the institution that was paying for it. I looked around and js/node was the clear winner. NodeJS was being continually improved with faster engines, the browser as the default GUI for any app I could create, Node can access to C libraries directly. So though I didn't know it at the time, I became full stack dev. And that is on top of my real job of engineer/data scientist. Unlike a lot of my colleagues who know only python/Fortran/C/C++ I'm comfortable putting together a server to crunch the numbers, tap in to a dB, and serve it all to a GUI of my own design if need be. JS/Node is not easy but it is the most flexible language ... oh, and you'll never be out of work even if that's the only language you know.