The English philologist and author J. R. R. Tolkien created several constructed languages, mostly related to his fictional world of Middle-earth. Inventing languages, something that he called glossopoeia (paralleling his idea of mythopoeia or myth-making), was a lifelong occupation for Tolkien, starting in his teens.
Tolkien's glossopoeia has two temporal dimensions: the internal (fictional) timeline of events in Middle-earth described in The Silmarillion and other writings, and the external timeline of Tolkien's own life during which he often revised and refined his languages and their fictional history. Tolkien scholars have published a substantial volume of Tolkien's linguistic material in the History of Middle-earth books, and the Vinyar Tengwar and Parma Eldalamberon journals. Scholars such as Carl F. Hostetter, David Salo and Elizabeth Solopova have published grammars and studies of the languages.
He created a large family of Elvish languages, the best-known and most developed being Quenya and Sindarin. In addition, he sketched in the Mannish languages of Adûnaic and Rohirric; the Dwarvish language of Khuzdul; the Entish language; and the Black Speech, in the fiction a constructed language enforced on the Orcs by the Dark Lord Sauron. Tolkien supplemented his languages with several scripts.
Context
Tolkien's hobby: glossopoeia
Tolkien was a professional philologist of ancient Germanic languages, specialising in Old English. Glossopoeia, the construction of languages, was Tolkien's hobby for most of his life.
One of the first constructed languages Tolkien encountered was "Animalic", invented by his cousins Mary and Marjorie Incledon. The language mainly used animal names, for example "Dog nightingale woodpecker forty" translated to "You are an ass". Tolkien's name in Animalic was "Otter". He learned a bit of it, but was not fluent in Animalic. Together with Mary, Tolkien invented Nevbosh, a sound substitution cypher known, 'new nonsense', based on English with French and Latin borrowings, which grew to include some elements of actual invented language. Shortly thereafter, he developed a true invented language called Naffarin, which used Latin and Spanish elements.
Tolkien was well aware of Esperanto, from c. 1907-1911, when he used it in one of his notebooks. Part of the lecture was published in The Monsters and the Critics, and Other Essays; in the part that was not, Tolkien gave the example of "Fonwegian", a language with "no connection whatever with any other known language".