Languages of Belgium
Before the federal structure and the language legislation gradually introduced in the 20th century, French was generally the only language used by public authorities. For example, the Dutch version of the Constitution has enjoyed equal status to the original French one only since 1967, and the German version only since 1991.
Of the inhabitants of Belgium, roughly 59% belong to the Flemish Community, 40% to the French Community and 1% to the German-speaking Community, though these figures relating to official Belgian languages include unknown numbers of immigrants and their children speaking a foreign language as primary language, and of Belgian regional migrants which may be assumed to largely balance one another for natively French and Dutch speakers. Though the standard form of Dutch used in Belgium is almost identical to that spoken in the Netherlands and the different dialects spread across the border, it is often colloquially called "Flemish".
Partition of Belgium
The main reason for Brussels being mainly French-speaking was the low social prestige of the Dutch language in Belgium during the 19th century and the severe discrimination of both Dutch as a language and of the Flemings. French was, at that time, the language of the administration, government, culture, law and education. From 1880 onwards, more and more Dutch-speaking people became bilingual and passed only French on to their children, resulting in a rise of monolingual French-speakers after 1910. Halfway through the 20th century, the number of monolingual French-speakers carried the day over the (mostly) bilingual Flemish inhabitants. Only since the 1960s, after the fixation of the Belgian language border and the socio-economic development of Flanders was in full effect, could Dutch stem the tide of increasing French use.