Russian language

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 (NCC-1701) encountered a Melkotian buoy in 2268, it spoke to the crew in their native languages, which included Russian, as heard by Pavel Chekov. 

Upon discovering that life signs had been detected in Spock's sealed quarters in 2285, Chekov exclaimed "Ya ne sumasshedshi! (Я не сумасшедший!)" ("I'm not crazy!")

When commissioned in 2363, the SS Tsiolkovsky's name was written in Cyrillic on her dedication plaque. 

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?) " (or "What the hell happened?").