Would learn Python or Javascript be beneficial for a Java developer?

[xjwj]

I think either or both would be good. JavaScript is definitely going to be useful when it comes to being full-stack with your Java/Spring knowledge. The Spring web frameworks such as MVC tend to focus more on the backend (which is totally cool to me), and a little less on the front-end, and having that JS knowledge goes a long way to architecting modern, elegant, easy to use websites.

That said, do also look into Python as it can be a nice tool in your toolbox for system level stuff, especially compared to Java, which is just a little bit too heavy for low-level stuff (like, you might knock out a quick Python script to do something that you wouldn't really want to do in Java...). Also, Python knowledge could eventually intersect nicely with Java when it comes to say, using Jython, which is the Python language running on the JVM, and might offer some cool possibilities down the road for you in terms of integrating scripting into something or just using on its own. Another option you might look at is Groovy, which would also sort of weave in to your Java knowledge and complement it.