Russian language
From Memory Alpha, the free Star Trek reference.
Russian was a language spoken on the planet Earth, originating in the nation-state of Russia. The alphabet it was written in was called "Cyrillic".
When the USS Enterprise encountered a Melkotian buoy in 2268, it spoke to the crew in their native languages, which included Russian, as heard by Pavel Chekov. (TOS: "Spectre of the Gun")
Upon discovering that a lifesign had been detected in Spock's sealed quarters in 2285, Chekov exclaimed "Ya ne sumasshedshi! (Я не сумасшедший!)" ("I'm not crazy!") (Star Trek III: The Search for Spock)
When commissioned in 2363, the SS Tsiolkovsky's name was written in Cyrillic on her dedication plaque. (TNG: "The Naked Now")
In the Julian Bashir, Secret Agent holoprogram, after waking up in Hippocrates Noah's secret lair, Anastasia Komananov says "Shto chort sluchilos'? (Что, чëрт, случилось?) " ("What the hell happened?") (DS9: "Our Man Bashir")
