The main language is Spanish, and for many people, it's the only language they speak. People in the tourism industry also speak English.
The Argentine peso is the main currency in Argentina. However, they have a lot of problems with inflation, where the value of the money drops very quickly. Because of this, many people are happy to be paid in U.S. dollars. Not everybody is, so you have to ask before you try to pay for something in U.S. dollars!
I do not buy plastic bottles of water, and neither should you! Too much plastic ends up in the ocean. Try carrying around a reusable water bottle, and you can fill it up in lots of different places! That's a simple thing you can start doing today.
I went to a restaurant called Tegui, which is one of the best restaurants in the world. They have a 14-course tasting menu meant to teach you a bit about the history of Argentina. It includes appetizer courses of green peas and fava beans, ricotta with fennel and edible flowers, scallops and unripe strawberries, heart of palm, oyster with bone marrow and seawater broth, sweetbreads (which is a fancy name for eating the pancreas!) with peaches and almonds, duck and roasted banana, blood orange, and plums with cream!
