French is an imported language in Brussels introduced during the Burgundian regime. French words were used in the city during the Middle Ages, when expressions such as allez, allez were spoken and written, but it was the arrival of a French-speaking aristocracy during the Burgundian regime that led to elevation of the language as the preferred speech of the upper classes. As such, acquisition of French became a mark of social and economic status. It gradually became the language of choice of the middle classes and received a significant boost during the French regime. On Belgian independence (1830) it became the official language of Belgium and the language of commerce, government, and educational instruction in Brussels. Use of French in the metropolitan area grew steadily through the 19th and 20th centuries. It is the language spoken by approximately 85 percent of native-born area residents.
Standard French is one of the three official languages of Belgium. Most immigrants from Wallonia to Brussels have spoken Belgian French, which is distinguished from the standard idiom by differences in pronunciation and vocabulary.
Historical Dictionary of Brussels. Paul F. State.