There are at least as many modern invented languages as there are natural ones, including some you might not think of as invented.
For instance, aspects of ancient languages like Hawaiian and Modern Hebrew are invented to bring them up to date, so that they can meet the demands of the twenty-first century. Other languages -- Cornish, Néo-Breton, and Neo-Galician, for instance -- are reconstructed for political and ideological reasons, to re-establish and reinvigorate national or ethnic identities.
In the sixteenth through eighteenth centuries, Neo-Latin revitalized Latin so that it could become an international language of science and, to a very limited degree, it still is one today -- newly discovered species of plant or animal get new Latin names.
In contrast, Volapük, Esperanto, Ido, Dil, Spokil, and dozens of other languages, were invented as "international auxiliary languages," structurally close enough to major natural languages for anyone to learn them easily.
Russian novelist Leo Tolstoy claimed to have learned Esperanto in two hours and, in the first decade of the twentieth century, ads for Esperanto textbooks promised mastery after a mere hour of study. Once all of us learn Esperanto or another international language, we will overcome the barriers erected by national languages, finally understand one another regardless of heritage or politics, and get down to the business of world peace. At least, that's the theory.
And then there are the many languages invented for fictional worlds, from the language supposedly spoken by inhabitants of the Moon in Francis Godwin's The Man in the Moone, published in 1638, to that of the Wards of Aghâr in Frédéric Werst's Ward, published earlier this year.
In the literature between them, you may have encountered
• Newspeak in George Orwell's 1984;
• Nadsat in Anthony Burgess' A Clockwork Orange;
• the extreme slang of Belinda Webb's recent feminist revision of Burgess' idea in A Clockwork Apple;
• the earlier feminist language called Laáden in Suzette Haden Elgin's Native Tongue and its sequels;
• and, of course, the several languages of Middle-Earth that appear in J. R. R. Tolkien's The Lord of the Rings and are accounted for more fully in the vast, underlying mythology published by Christopher Tolkien in twelve volumes as The History of Middle-Earth;
• not to mention Klingon in Star Trek's Klingon and, more recently, Avatar's Na'vi.
Invented languages are perfect for science fiction and fantasy -- they force us to think in cultural terms different from our own.
As the literary critic Eric Rabkin has argued, fantasy challenges "the very nature of ground rules, how we know things, on what bases we make assumptions." When we encounter an invented language in literature, we focus, not only on the language in its fictional context, but on the very nature of language: we confront and admire its world-shaping power, a power vested in both invented languages and natural ones, indeed, in the very languages real speakers use in the real world every day.
Here are eight examples of fantastical, fictional languages. How many can you speak?