I tend to haxe for my side projects and it sounds like in principle it's not far off what you're looking for.
The haxe compiler outputs js, python, c++, java, c#, php (along with a bunch of others) and bares a lot of resemblance to c# or java. For personal projects it's perfect - write once, run natively everywhere, and with a single codebase for client and sever.
But there are good reasons not to use it too. Its community is still fairly small and highly game-dev-centric so you'll probably have trouble finding the sort of high quality data science libraries you might find with a language like Python. That being said, haxe can take advantage of libraries on its target platform, so if you're compiling haxe to python, all the python libraries are available to you. The next problem with haxe is that it's the job market for it is still very small compared to the other major languages.
So it's ideal for personal projects, where you just want to get as much done as possible without being bogged down by platform specifics, but unless you're in game dev, you're going to struggle to get employment from it.
I agree with anon3_ that Python's a sensible choice for employability and compatibility. Node.js and ES6 are worth a look into too, especially if you're looking to get into server-side work. Because of its popularity, there's an awful lot of weight behind javascript and it doesn't look like it's easing up any time soon.