(written from a Production point of view)
Marc Okrand (born 1948) is the inventor of the Klingon language. He initially created the Vulcan dialogue in Star Trek II: The Wrath of Khan. He was hired by Paramount Pictures to invent the language and coach the actors on Star Trek III: The Search for Spock, Star Trek V: The Final Frontier and Star Trek VI: The Undiscovered Country. Okrand's basis for the Klingon language came from the few samples spoken in Star Trek: The Motion Picture which were made up by James Doohan. He is also famous as the author of The Klingon Dictionary and all its addenda.
On Disc 2 of the Star Trek III: The Search for Spock DVD, Okrand said he was asked to create Klingonese for scenes that had already been shot in English. During post-production, he turned off the sound and observed the actors' lip movements as they spoke their lines. From these movements, he developed new vocal "sounds" for the actors to dub over their original English. He was asked to do the same for Spock and Saavik in Star Trek II: The Wrath of Khan, during their brief discussion about Kirk in the Torpedo Bay. Okrand was also interviewed on 21 August 1991 for the TNG Season 5 DVD special feature "Alien Speak" ("The Klingon Linguist").
Okrand previously worked with Native American languages, from which he borrowed the unusual Klingon tlh [IPA: /t͡ɬ/] sound (common in North and Central American indigenous languages, in which it is usually transcribed as tl; this is the sound at the end of the word "Nahuatl" as the Aztecs pronounced it themselves).
Recent work includes the Atlantean language which he was hired to do for Disney's film Atlantis: The Lost Empire, which featured the voice of Leonard Nimoy. He also contributed Klingon and Vulcan dialogue for several episodes of the fourth season of Star Trek: Enterprise. He was a consultant for the Klingon, Romulan and Vulcan languages on 2009's Star Trek and served as an uncredited consultant on Star Trek Into Darkness.