JavaScript is a decent option. If you've narrowed it down to Python and Java, as a long-time Java programmer, I recommend Python. Java has extra toil that makes sense for its professional use cases but get in the way of learning for beginners. Python is easy to get started with because there's no compiler — "hello world" is just installing Python and typing two lines into a text file. Plus

ipython

is a nice interactive environment for experimenting, which is roughly comparable to running JavaScript interactively in a browser console.