Swahili language

Swahili was a language spoken on the planet Earth, originating in the African continent, and belonging primarily to the Swahili people of East Africa.

A mysterious crewman encountered by Uhura, who was (unknown to her) the M-113 creature in Human form, spoke to her in Swahili. He stated "Ninakudhania mwanamke mzuri," which meant "I think of you, beautiful lady."

In 2267, Uhura had her memory erased by the Nomad space probe, forcing Uhura to be reeducated. While working with Nurse Chapel, Uhura had difficulty sounding out the words in her first grade reader, and began speaking in Swahili out of frustration, saying "Sikumbuka" ("I don't remember"). Chapel then encouraged her, by telling her to speak it in English, not Swahili, and proceeded to sound out the sentence for her. 

When the USS Enterprise (NCC-1701) encountered a Melkotian buoy in 2268, it spoke to the crew in their native languages, which included Swahili, as heard by Uhura.