While I'm a big fan of being able to write mobile applications in JS, the reality is that it still has to be transpiled into Java and compiled into native code. At least that's my understanding, which I'll admit may be out of date... I haven't done this in a while. I don't really see JS taking over, though it would be nice if Google would let JS apps run natively, especially since they have their own fast JS engine. To my knowledge, this isn't supported quite yet.
As for Python, it's fine for a back-end, but as others mentioned, doesn't do anything on the device itself. There are a couple of solutions out there that let you do Python development for Android, such as Kivy, but since Android doesn't include a Python interpreter, you'd have to bundle one with your app, and that will both inflate the size of your app and slow it down.