The first time the Klingon language is spoken in Star Trek might surprise you. The existence of the Klingon language was first mentioned in Star Trek: The Original Series' classic episode "The Trouble With Tribbles". Klingon commander Korax (Michael Pataki) taunts Montgomery Scott (James Doohan) by comparing the USS Enterprise to a garbage scow and insisting that "half the quadrant [is] learning to speak Klingonese". This implies that Star Trek: The Original Series' Klingons, like Kor (John Colicos) and Koloth (William Campbell), can speak English with ease, instead of relying on the Universal Translator to communicate with Starfleet officers.
Since then, the Klingon language has appeared in Star Trek with astonishing regularity. Klingon phrases have been in nearly every Star Trek show since Star Trek: The Next Generation. Sometimes, like in Star Trek: Deep Space Nine, the spoken Klingon isn't subtitled, making these scenes a treat for Star Trek fans who have studied the Klingon language. But the Klingon language didn't spring into existence fully-formed, and it certainly wasn't always the cultural phenomenon that it is today. In fact, not a single word of Klingon was spoken during Star Trek: The Original Series at all.
The Klingon Language Is Spoken For The First Time In Star Trek: The Motion Picture
But Klingon Wasn't Fully Developed Until Star Trek III: The Search For Spock
We hear the Klingon language spoken for the first time in Star Trek: The Motion Picture (1979). The words used in the film were created by James Doohan and associate producer Jon Povill. Along with the Klingon makeup redesign, the few phrases in the Klingon language from The Motion Picture were enough to define Klingons as more distinctly alien than in Star Trek: The Original Series. Later, when Star Trek III: The Search for Spock needed a more robust Klingon language, linguist Marc Okrand developed a bigger vocabulary based on Doohan's phrases, and codified Klingon syntax.
Given the state of relations between the Klingon Empire and the Federation at the time of Star Trek: The Original Series, it's not surprising that most Starfleet officers weren't well-versed in speaking Klingon. Because Star Trek is told from the Federation's point of view, we start to hear more Klingon spoken in Star Trek as the Federation learns more about Klingon culture. The fact that the Klingon language was first spoken in Star Trek: The Motion Picture is, perhaps unintentionally, foreshadowing how the Klingon Empire and the Federation later broker peace in Star Trek VI: The Undiscovered Country.