Short answers, pulled from the story.
A constructed language, often shortened to conlang, is a language intentionally devised by a person or group for a particular purpose, rather than emerging naturally from human interaction. The term encompasses subcategories including fictional, artificial, engineered, planned, and invented languages. Conlangs may include phonology, grammar, orthography, and vocabulary.
The Lingua Ignota, recorded in the 12th century by St. Hildegard of Bingen, is considered apparently the first entirely artificial language. An earlier literary reference to the concept appears in the seventh-century Irish text Auraicept na n-Éces, which describes a scholar named Fénius Farsaid combining the best features of many languages to create one new tongue.
According to Ethnologue, between 200 and 2,000 people speak Esperanto as a first language. The Hungarian census of 2011 counted 8,397 speakers of Esperanto overall, making it by far the most widely spoken constructed language.
Klingon was created by Marc Okrand for the Star Trek franchise in 1985. In a 2015 lawsuit involving a fan film called Axanar, the Language Creation Society argued that Klingon is not copyrightable under section 102(b) of the Copyright Act of 1976 because it is "a procedure, process, or system for communication" rather than an expression of an idea. The legal status of language ownership remains unresolved.
An a priori constructed language has features not based on any existing language, while an a posteriori language borrows its elements from existing languages. The distinction is not always absolute, as a single language may be a priori in some respects and a posteriori in others. Most international auxiliary languages are a posteriori because borrowing from existing languages makes them easier for speakers of those languages to learn.
Volapük was a constructed international auxiliary language proposed by Johann Martin Schleyer in 1879. Within a decade it attracted 283 clubs worldwide. Disagreements between Schleyer and prominent users of the language caused a schism, and by the mid-1890s it had fallen into obscurity, giving way to Esperanto, which Zamenhof had published in 1887.