German is the official language of Germany, with many people also speaking English, Spanish and various other languages. Where I live on the eastern border, Polish is also widely spoken.
The euro was adopted in Germany alongside many other EU countries in 2002. Before then, Germany used a currency known as the Deutsche "Mark".
Water bottle costs can vary greatly if you're in the supermarket versus a tourist shop. In the supermarket, you can expect to pay around 60¢ or €1. In more touristy areas you'll find one for €3-4
While not overly German, my friend made me a traditional Sunday Pot Roast to give me a small taste of home. We enjoyed scouring the supermarket looking for ingredients I typically don't look for in my day-to-day life, such as chuck roast and cornstarch.
