Spanish is a language spoken on Earth.
Both Jack O'Neill and Daniel Jackson were able to converse in Spanish while in Honduras in 2003, though it is unclear how well either of them spoke the language. (SG1: "Evolution, Part 1", "Evolution, Part 2")
A United States Air Force linguist named Lindsay said that she was fluent in this language, when applying for a post at SG-1 in 2005 after Cameron Mitchell assumed command over the unit. (SG1: "Avalon, Part 1")