On tour, I just had the privilege of some much needed reading time and I was able to breeze through Arika Okrent's highly entertaining "In the Land of Invented Languages". She spends long chapters on the curiously durable Esperanto, the bizarre but beautiful Blissymbolics, and the befuddling Loglan.
The first "invented" language though, was the work of Hildegard of Bingen, the noted Medieval Christian mystic. Meaning "Unknown Language" in Latin, Hildegard devised a vocabulary of about 1000 words, and a corresponding unique orthography of her own invention. With terms like "phazur" (grandfather) and "scirizin" (son), the language almost appears to be a mixture of Latin and German with a bit of Basque.
There has been speculation as to the possibility that Hildegard had designed the language for Utopian purposes, much as 19th century language inventors such as Esperanto's Zamenhof. But the stronger likelihood is that she intended it to be a sort of secret language, perhaps for divine communication.