In life, we are the servants of language. Words are all we leave behind us. What substances we leave are only significant by the names they receive. It is the so named Works of the Romans, not any work alone, that inspires wonder—and it is only the name Work which causes us to regard the aqueduct at Nîmes as more wonderful and more attention-worthy, more sympathetic, than any arch of weatherworn stone in the Badlands of Utah. The name of Apelles the painter was a byword for excellence in his art for a thousand years after the last man who had seen his pictures was dead.
Language is all that we have in common. What we make are only words; what we leave, are only names. The side effect of world-spanning intelligence is that each intellect becomes its own world—a principle compelling diversity, which only the necessity of communication, restrains from dividing and ruinating our species. Therefore, what can be said in any language can always be said in another—because while we are under no obligation to be able to communicate a thought, what can be communicated to anyone, can be communicated to everyone. Think of how strange it is, that only because you both speak the same language, you can communicate with someone separated from you by decades of age, by country and climate, by sex, by class, by way of life. And think how absurd it is, that someone of your age, sharing your background and your circumstances, cannot communicate across that shorter distance, if you do not share a language. Yet, it is possible to know things in one language which you do not know in another: not because they cannot be said in both; but because translation is a kind of alchemy, where solve must precede coagula. What you take in whole need be accessible to you in another language, even if you speak them both, until you have taken it apart in one language, to put it back together in another.
What then could you not learn, by creating a language? Let the language of mathematics bear witness. But my concern for now is with languages created for pleasure, not for purpose.
Tolkien is the master—the Old Master—of the artistically constructed language. Much of the appeal of reading Tolkien is to discover things that he knew, in silver-quick Quenya and in Sindarin's rushy breezes, that he did not quite know in English, and could not always translate. Indeed, On Fairy-Stories has the air of a poor translation from a patois of Elvish languages inside his head. And A elbereth gilthoniel silivren penna míriel teaches you something without having to see a translation at all; but you can feel that you know it, whatever it means, under branches screening stars.
Tolkien is sometimes derided as pseudo-Biblical in his diction; but this is simply wrong. The peculiar cadence of the King James Version is imparted by a plangent alternation between etymologically disparate English synonyms—which misrepresent the straightforwardness of the Hebrew. But Tolkein is etymologically almost pure: page after page, he goes on in a kind of alternate or underground English, the revenant language of Chaucer, the sleek hull of maiden-voyaging English before it was barnacled with borrowings from Latin and Greek. This language—a constructed language of a sort, purely by selection—is, before even myth and archetype, the deepest source and means of his power.
Some purposes require their own languages. Mature poetry has its own grammar; vital religions employ their own dialects, even their own languages (and a wise missionary does not translate everything); and every profession must find its own language, to ennoble the commonplace or to make commonplace the extraordinary. And every language supports another, floating language, of idioms and proverbs—one which can sometimes be carried over whole into new waters, as Erasmus did in his Adagia, which restored to the Renaissance the floating language of the ancient world.
Art, insofar as it is mimetic, is still only sometimes the recapitulation of a natural process. As often, artistry is the power to throw a natural process into reverse. A picture suggests a story; a title finds a poem; a stain on a wall evokes a picture. The constructer of a language only begins by finding, in the space between real languages, the pleasing or striking form of a new and unheard language. The art is to evolve, from this shadow, the succession of necessities comprising the thinkable history that would have formed that language—that would have learned all that that language knows.