“Si sólo hablásemos cuando tenemos algo que decir, el uso del lenguaje desaparecería en dos generaciones.” -Noel Clarasó
(“If we were to speak only when we have something to say, the use of language would disappear in two generations.”)
If you’re asking why someone should learn another language on top of what they’ve picked up from birth (common to the arrogant, self-centered variety of native English speakers), I’d ask you to backtrack a bit and tell you why you should study any language at all. Language is the building block of all humanity, the masterpiece of the rationality bestowed so uniquely upon no one else but us. Language is an art, as art is the exaltation of the human soul. Language is a philosophy, as philosophy is a passion for the secret wisdom of life. To study language is to get closer to that which makes us human. The study of anything leads us to understanding, and places us in a better position from which to interact with all of creation. In this case, understanding language is a hinge from which all axes of movement towards our fellow human beings is valued as due. That is why it is anyone’s duty to study language.
A beginner foreign-language learner might not be able to appreciate the above statement in its full glory until he’s learned his third or fourth language. Don’t worry. The secret beauty of language does not change. It will wait for you when you are ready to celebrate it properly. For now, think about it this way: there is no better way to open your mind to the wisdom of the world than to learn languages. I would say the same of traveling, but I would insist that the two are inseparable. Travel is of the physical realm, for the body and the senses, while language-study is travel for the mind.
Besides the obvious, practical reasons, of which I think nobody needs convincing, picking up a new language is not just to be able to communicate something using different sounds and words. As the quote above says, if it were only for speaking, language would die quickly. This is because language is alive. It carries with it the secrets, joys, fears, tears, and hopes of a people — from a time long inaccessible to you by the separation of years past, to a time which you will not live to see, in the generations of the future. Language is a vehicle, it moves, it carries, it dances, it sings. Yes, it is alive. Each language is a character with a face, with a shape, with a voice. It has stories to tell. It sees the world in a way that is unique only to it. The beauty in the plan is that this wisdom is not hidden from us. We are put into the world already equipped to share in that secret. One only needs to take the plunge, to dive in a language, or two, or three.
Think about it carefully.