The AHD gives the Proto-Indo-European root as *stom-en- "Denoting various body parts and orifices". In Greek it's στόμα "mouth", whence στόμαχος "throat, gullet" then "stomach", and then borrowed as Latin stomachus, Old French stomaque, borrowed as English stomach. The English word was first spelled stomak/stomack, it seems that it was later respelled stomach after the Latin spelling.
In Celtic it became Middle Breton staffn, modern Breton staoñ "palate", Welsh safn "mouth, jaws, palate", and Cornish sâwn "cleft, fissure, ravine". The Cornish word was borrowed as zawn, a fissure or cave in a coastal cliff.