Russian, a language spoken in Russia on Earth. Daniel Jackson speaks Russian, which caused SG-1 to be suspected of Soviet espionage when they were sent back in time to August 4, 1969. (SG1: " 1969")
History[]
In 2005, Dr. Lindsay claimed that she was fluent in this language, when applying for a post at SG-1 after Cameron Mitchell assumed command over the unit. (SG1: "Avalon, Part 1")