JS is not a sketchy mess, but it has been used to build a lot of sketchy messes. That was far more true a decade ago than it is today. In 2015 it is just as easy to write concise, correct, and maintainable code in JS as in any of its common competitors. If you want to learn something very practical, JS and its vibrant ecosystem is absolutely what you should learn. You will likely be frustrated initially, but come out the other side with a very valuable tool for your toolbox and a new respect for the language. Check out ES6 as well, it is a very nice language that is currently transpiled to JS like Typescript or Coffeescript, but it may eventually be implemented in browsers directly.

If you are looking for something less practical, but perhaps more interesting to your backend sensibilities and less frustrating, I would recommend Elixir. It's a really great language running on the really great Erlang VM, which I envision being a really great alternative to the usual suspects of the backend. I'm using it to build an API-based service, and have found it extremely fun to work with and refreshingly easy to get great performance (after doing Rails for many years).

Edit to add: Also, since you already know (and seem to like?) Java, learning how to build Android apps is both fun and practical, and you could port your app to iOS for an easier way to learn Swift, which is another great language that will definitely be a great bullet point on a resume for years to come.

Many thanks, I've always been attracted to Erlang, so I'll definitely have a look to Elixir