Those that know me know that I'm a real programming language geek. I'm always reading about programming languages. Why am I constantly seeking out new language while most other developers are happy knowing one or two languages? I think it comes down to not being satisfied with the status quo. When you code in a language for a long enough period of time you think in terms of the constructs that the language provides you.
This is certainly a good thing in terms of productivity. Thinking in a language is akin to speaking it as your native tongue. Your brain becomes hard wired to think in that way. The time from concept to implementation is shortened because you don't have to do any mental translation to convert a solution from one language to another. While this is all well and good, thinking only in the constructs of one language prevents you from seeing . As the old saying goes: when all you have is a hammer, every problem looks like a nail.
But while this is great in some respects, learning more languages has certain benefits. Each language is different, exposing you to different ways of approaching problems and different tools for solving them. As you step further and further away from the comfortable object oriented Aglol descendants you'll notice that the languages you used to use to solve every problem is woefully inadequate at solving certain classes of problems.
This comes from the fact that not all languages are created equal. Each language has a certain level of friction for a given problem domain. It's true that you can solve a problem in any turing complete language, but that doesn't mean that the solution will be easy to read, write, or maintain. By learning a variety of languages you'll be able to see which language are better suited to a particular problem.
Also, since each language tends of offer a different set of tools, you'll expand your mind. There is more to programming than what blub languages give you. For example, JavaScript gives you a language with dynamic typing, first class functions, closure, and prototypal inheritance. None of those features are available in a language like Java.
Features aren't the only thing that new languages expose you to either. As you venture further off of the beaten path you'll find some very interesting languages that look strange and alien. Take Factor for example. Factor is a concatenative language which is a language that is based around the idea of a mutable stack. Concatenative languages are just one class of languages and there are many others, all of which have interesting properties.
As you become exposed to more and more language concepts you'll be able to pick up new languages much more easily. Having been exposed to the concepts of a new language before, you will have flattened the learning curve of that language.
The most effective languages tend to be called Domain Specific Languages. A domain specific language is a language that is tailor made for solving problems in a specific domain. A great example of a domain specific language that most of us have dealt with is CSS. CSS provides a language to specifying styling of HTML elements. It doesn't have loops, it doesn't have conditionals, just pure unadulterated declarations that can be combined. Certainly there are problems with implementations of the language, but that isn't my point. My point is that CSS fills a void. You could probably do what CSS does with a general purpose language, but why? CSS is tailored to the problem it's trying to solve.
I'll leave you with an interesting thing I've noticed as more applications are moved into the web realm: if you want to stay relevant, you need to become a polyglot. Most applications aren't simply constrained to one language anymore. Modern web applications use at least 4 distinct languages between the server side, client side, build scripts, and configuration. If you can't be effective with all of them then you've got some learning to do.
