The Klingon dialogue wasn’t originally used in The Motion Picture
Called Klingonese in the Star Trek: The Original Series episode, “The Trouble with Tribbles,” the Klingon language was never spoken onscreen until it debuted in the 1979 movie, Star Trek: The Motion Picture. And, originally, it wasn’t spoken in that movie, either.
When The Motion Picture was in production, the actors who played the Klingons said their lines of dialogue in English. It was only after the scenes were recorded that it was decided the dialogue should be in Klingon. James Doohan (who portrayed Commander “Scotty” Montgomery Scott), who had an ear for languages and dialects, along with associate producer, Jon Povill, took on the challenge of creating the words that were later spoken by the actors. And it was much more of a challenge than fans ever knew as Doohan and Povill had to come up with words that would fit the actors’ existing lip movements so the lines only had to be dubbed and not refilmed.
And then the Klingon language continued
Though Doohan started the language, linguist Dr. Marc Okrand developed it. Actor Mark Lenard, who portrayed Spock’s father, Sarek, made sure Dr. Okrand knew who’d created the first lines of the Klingon language, though, and before Okrand, Lauren Weinstein, who worked in the effects department for The Motion Picture, was given the task of creating a written Klingon language.
Klingons have routinely spoken English on Star Trek series and in the films, but the addition of a language they can call their own added to their originality. It’s certainly a difficult language to speak, although, there are thousands upon thousands of people still trying to learn it. In fact, so many people wanted to learn the language, Duolingo began offering free training in 2018 that still continue to this day.