The Dutch word for stomach is
maag

Dutch Definition

maag	©
1. n-f. (topics, Organs) stomach, gut

buik
1. n-m. (topics, Body parts) belly
Hij heeft een grote buik. - He has a big belly.
2. n-m. paunch (referring euphemistically to a protrusive belly)
3. n-m. (nautical) The lowest inner part of a ship's hull, where water accumulates.
De zeilboot heeft water in de buik. - The sailboat has water in its hull.