There are three officially recognized languages: French and Dutch are the main languages, German is spoken by a small segment of the population and English is widely spoken. Most signs in Brussels are in English, French and Dutch. Do not hesitate to start a conversation in English...most of the Belgian people speak it fluently, with that charming french or dutch accent.
Note that what is called the first floor in the United States is called the ground floor in Europe--the American second floor is the European first floor, the American third floor is the European second, and so on. Also, when ordering in a restaurant, what Europeans call the entree is the first course or appetizer of a meal; the main course is called le plat principal (French) or the hoofdgerecht (Dutch).