In Luxembourg, we already talked about how most people everywhere in the country speak at least French, German, and Luxembourgish. In Belgium, however, the languages spoken are divided by geography. Dutch, French, and German are all spoken, though Dutch is spoken in in Flanders (the North of Belgium), French is spoken in Wallonia (the South of Belgium), and German is spoken by a small sector of Eastern Belgium. The divisions are quite interesting; be sure to look up a map and see!
Because both Luxembourg and Belgium are countries within the European Union, they both utilize the euro € as their official currency. These come in coins and paper dollars, but most people just use their credit cards, Google Pay, or Apple pay.
A bottle of still water in Belgium can cost anywhere from 2.00 to 5.00 euros. This equals about 2.32-5.80 U.S. dollars. Because Luxembourgers and Belgians both prefer sparkling water over still water, however, this is subject to change based on brand.