The main language spoken in Madrid is Spanish (or Castilian). Spanish is also the main language throughout Spain. There are other languages that are widely used in Spain too, such as Catalan, which is spoken in Barcelona, and Galician, which is spoken in Galicia.
The Euro is used in Spain. You might have heard of other countries that use the Euro as well. Many countries in Europe are part of a group called the European Union. Every country that belongs to the European Union uses the Euro as their money. They use coins and paper as well. The symbol for the Euro is €. Coins are either one Euro, two, five, ten, 20 or 50. Paper money starts at five Euros, then ten, 20, 50, 100 and so on.
A bottle of water will vary in cost. At a small convenience store, a 1.5-liter bottle of water will cost one Euro. At restaurants; however, a bottle of water can cost you two and a half Euros. It's inexpensive when bought outside of restaurants.