One can hear just about every European language spoken in Brussels, plus languages from around the world. In the north of Belgium, called Flanders, the official language is Dutch. In the south of the country, called Wallonia, the official language is French. The city of Brussels is officially bilingual, with French and Dutch used for all contacts with the government.
At night, Brussels is predominately French-speaking, as more than eighty percent of the residents of the Brussels Region are francophone. However, by day the city is much more bilingual as many people in the Flemish periphery commute to work in Brussels.
Actually, it is even more complicated than that. German is also an official language of Belgium, and it is spoken in the Eastern Cantons, near the German border. However, you are likely to meet many German-speaking Belgians in Brussels.
Almost everyone speaks at least some English, and American students are often dazzled by the linguistic skills of the Belgians and other Europeans that they encounter. So, if you want to practice your French, Dutch, Spanish, Italian, etc., you have the opportunity in Brussels. However, you should not worry if you do not know another language. Everyone in Brussels likes to practice their English.
Street signs offer a variety of languages in Belgium