Russian (or Cyrillic) was a language spoken on the planet Earth, originating in the nation-state of Russia.
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")
When commissioned in 2363, the SS Tsiolkovsky's name was written in Russian 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 sloo-chee-luhs?" ("What the hell happened?") (DS9: "Our Man Bashir")