Around the year 1000, the County of Brussels became a part of the Duchy of Brabant (and therefore of the Holy Roman Empire) with Brussels as one of the Duchy's four capitals, along with Leuven, Antwerp, and 's-Hertogenbosch. Dutch was the sole language of Brussels, as in the other three cities. However, not all of Brabant was Dutch-speaking. The area south of Brussels, around the town of Nivelles, was a French-speaking area roughly corresponding to the modern province of Walloon Brabant.^[37]

Initially, Latin was used as an official language in Brussels, just like in most of Europe. From the late 13th century, people began to shift usage to the vernacular. This occurrence took place in Brussels and then in other Brabantian cities, which had all eventually transformed by the 16th century. Official city orders and proclamations were thenceforth gradually written in Middle Dutch. Until the late 18th century, Dutch remained the administrative language of the Brussels area of the Duchy of Brabant. As part of the Holy Roman Empire, Brabantian cities enjoyed many freedoms, including choice of language.^[37] Before 1500, there were almost no French documents in the Brussels city archives. By comparison, in the cities in the neighboring County of Flanders such as Bruges, Ghent, Kortrijk and Ypres the percentage of French documents in city archives fluctuated between 30% and 60%. Such a high level of French influence had not yet developed in the Dutch-speaking areas of the Duchy of Brabant, including Brussels.^[37]

After the death of Joanna, Duchess of Brabant, in 1406, the Duchy of Brabant became a part of the Duchy of Burgundy and the use of the French language slowly increased in the region.^[38] In 1477, Burgundian duke Charles the Bold perished in the Battle of Nancy. Through the marriage of his daughter Mary of Burgundy (who was born in Brussels) to Holy Roman Emperor Maximilian I, the Low Countries fell under Habsburg sovereignty. Brabant was integrated into this composite state, and Brussels flourished as the capital of the prosperous Burgundian Netherlands, also known as the Seventeen Provinces. After the death of Mary in 1482, her son Philip the Handsome succeeded as Duke of Burgundy and Brabant. In 1506, he became the king of Castile, and hence the period of the Spanish Netherlands began.

After 1531, Brussels was known as the Princely Capital of the Netherlands. After the division of the Netherlands resulting from the Eighty Years' War and in particular from the fall of Antwerp to the Spanish, Netherlands' economic and cultural centers migrated to the northern Dutch Republic. About 150,000 people, mainly stemming from the intellectual and economic elites, fled north.^[39] Brabant and Flanders were engulfed in the Counter-Reformation, and the Catholic priests continued to perform the liturgy in Latin.

Dutch was seen as the language of Calvinism and was thus considered to be anti-Catholic.^[38] In the context of the Counter-Reformation, many clerics of the Low Countries had to be educated at the French-speaking University of Douai.^[40] However, Dutch was not utterly excluded in the religious domain. For instance, Ferdinand Brunot reported in 1638 that, in Brussels, the Jesuits "preached three times a week in Flemish and twice in French".^[40] While Dutch became standardized by the Dutch Republic, dialects continued to be spoken in the south.^[27] As in other places in Europe during the 17th century, French grew as a language of the nobility and upper class of society.^[41][42] The languages used in the central administration during this time were both French and, to a lesser extent, Spanish.^[37] Some French-speaking nobility established themselves in the hills of Brussels (in the areas of Coudenberg and Zavel), bringing with them primarily French-speaking Walloon personnel. This attracted a considerable number of other Walloons to Brussels who came in search of work. This Walloon presence led to the adoption of Walloon words in the Brussels flavor of Brabantian Dutch, but the Walloon presence was still too small to prevent them from being assimilated into the Dutch-speaking majority.^[37]

Following the Peace of Utrecht, Spanish sovereignty over the Southern Netherlands was transferred to the Austrian branch of the House of Habsburg. This event started the era of the Austrian Netherlands.

In the 18th century, there were already complaints about the waning use of Dutch in Brussels, which had been reduced to the status of "street language".^[44][45] There were various reasons for this. The Habsburgs' repressive policies after the division of the Low Countries and the following exodus of the intellectual elite towards the Dutch Republic left Flanders bereft of its social upper class. After the 17th century's end, when the Dutch Golden Age ended and the Dutch Republic began declining, Dutch lost even more prestige as a language for politics, culture, and business. Meanwhile, French culture was quickly spreading.^[2] For instance, the Theater of La Monnaie showed 95% of plays in French by the mid-18th century.^[40] During the War of the Austrian Succession, Brussels was under French rule between 1745 and 1749.^[37] Under these circumstances, especially after 1780, French became the adopted language of much of the Flemish bourgeoisie,^[37] who were later pejoratively labelled Franskiljons (loosely: little Frenchies). The lower classes got progressively poorer, and, by 1784, 15% of the population was in poverty.^[44] The small French-speaking minority was quite affluent and constituted the social upper class.^[46]

The percentage of the Brussels population using French in public life was between 5 and 10% in 1760, increasing to 15% in 1780.^[40] According to authenticated archives and various official documents, it appears that a fifth of municipal declarations and official orders were written in French. Twenty years later this rose to a quarter; however, over half of the official documents in French originated in the French-speaking bourgeoisie, who made up just a tenth of the population. In 1760, small businesses and artisans wrote only 4% of their documents in French; by 1780 this had risen to 13%.^[47] In private life, however, Dutch was still by far the most-used language.^[40][47] For the Austrian Habsburg administration, French was the language of communication, although the communiqué from the Habsburgs was seldom seen by commoners of Brussels.^[37]

Following the campaigns of 1794 in the French Revolutionary Wars, the Low Countries were annexed by the French First Republic, ending Habsburg rule in the region. The Flemish were heavily repressed by the French, who instituted heavy-handed policies that completely paralyzed the economy. Within this period of systematic exploitation, about 800,000 inhabitants fled the Southern Netherlands,^[48] and the population of Brussels decreased from 74,000 in 1792 to 66,000 in 1799.^[49] The French occupation led to further suppression of Dutch across the country, including its abolition as an administrative language.^[46][49] With the motto "one nation, one language", French became the only accepted language in public life, as well as in economic, political, and social affairs.^[50] The measures of the successive French governments, and in particular the 1798 massive conscription into the French army, were particularly unpopular within the Flemish segment of the population and caused the Peasants' War.^[51] The Peasant's War is often seen as the starting point of the modern Flemish movement.^[52] From this period until the 20th century, Dutch was seen in Belgium as a language of the poor and illiterate.^[45] In Flanders, as well as other areas in Europe, the aristocracy quickly adopted French.^[46][53] The French occupation laid the foundations for a Francization of the Flemish middle class aided by an exceptional French-language educational system.^[54]

At the beginning of the 19th century, the Napoleonic Office of Statistics found that Dutch was still the most frequently spoken language in both the Brussels arrondissement and Leuven. An exception included a limited number of districts within the City of Brussels, where French had become the most used language. In Nivelles, Walloon was the most spoken language.^[40] Inside of the Small Ring of Brussels (the Pentagon), French was the leading language of street markets and districts such as the Coudenberg and the Sablon/Zavel,^[55] while Dutch dominated in the harbor, the Schaerbeek Gate and the Leuven Gate areas. The first city walls were gradually dismantled during the 15th century to the 17th century, and the outer second walls (where the Small Ring now stands) were demolished between 1810 and 1840, so that the city could grow and incorporate the surrounding settlements.^[56]

Immediately after the French capture, the use of Dutch was forbidden in Brussels' Town Hall.^[47] The Francization policy, instituted to unify the state, were aimed at the citizens who were to assume power from the nobility as was done in the French Revolution.^[47] However, the French rulers rapidly understood it was not possible to force local populations, speaking languages very different from French, to suddenly use it. The Francization of the Dutch-speaking parts of the Low Countries, therefore, remained limited to the higher levels of the local administration and upper-class society.^[40] The effect on lower social classes, of whom 60% were illiterate,^[47] was small.^[41] Life on the streets was greatly affected as, by law, all notices, street names, etc. were required to be written in French,^[47] and official documents were to be written solely in French, although "when needed", a non-legally-binding translation could be permitted.^[41] Simultaneously, businesses from the rural areas were told not to continue operating if they were not proficient in French.^[40] In addition, the law stated that all court pleas, sentences, and other legal materials were to be written solely in French unless practical considerations made this impossible.^[40] These measures increased the percentage of official documents written in French from 60% around the start of the 19th century to 80% by 1813. Although mainly used in higher social circles, a more appropriate measure of actual language use might include an observation of written testaments, three-fourths of which in 1804 were written in Dutch, indicating that the upper classes still mainly used Dutch around the start of the 19th century.^[40]

In 1815, following the final defeat of Napoleon, the United Kingdom of the Netherlands was created by the Congress of Vienna, joining the Southern Netherlands with the former Dutch Republic. Shortly after the formation of the new kingdom, at the request of Brussels businesses, Dutch once again became the official language of Brussels.^[47] Nevertheless, the union of the Netherlands and Belgium did little to lessen the political and economic power of French in Flanders, where it remained the aristocracy's language.^[57] Brussels and The Hague were dual capitals of the Kingdom, and in the Parliament, the Belgian delegates spoke only French. King William I of the Netherlands wanted to develop present-day Flanders to the level of the Northern Netherlands, and instituted a wide network of schools in the local language of the people.^[37][46] He made Dutch the single official language of the Flemish provinces, and this was also implemented in bilingual Brabant and Brussels. The Walloon provinces remained monolingually French.^[47] The king hoped to make Dutch the sole language of the nation, but the French-speaking citizenry, the Catholic Church, and the Walloons resisted this move.^[46] The French-speaking population feared that their opportunities for participation in government were threatened and that they would become unneeded elements of the new kingdom. Under pressure from these groups, in 1830, the king reintroduced a language freedom policy throughout all of present-day Belgium.^[58][59] This nullified the monolingual status of Brussels and the Flemish provinces.^[47]

Important for the later development of the Dutch language was that the Flemish population experienced a certain amount of contact with the northern Standard Dutch during the kingdom's short reign.^[41] The Catholic Church viewed Dutch as a threatening element representative of Protestantism, while the Francophone aristocracy still viewed Dutch as a language subordinate to French.^[59] These views helped contribute to the Belgian Revolution and to the creation of an independent and officially monolingual Francophone Kingdom of Belgium, established in 1830.^[41][49][58] This strong preference for French would greatly influence language use in Brussels.

After the Belgian Revolution, the bourgeoisie in Brussels began to increasingly use French. Numerous French and Walloon immigrants moved to Brussels, and for the first time in mass numbers the Flemish people began switching to French.^[38][60]

By October 16, 1830 King William I had already rescinded a policy that named Dutch as the official language of Brussels.^[61] The sole official language of the newly created centralized state was French, even though a majority of the population was Flemish.^[46] French became the language of the court, the administration, the army, the media, and of culture and education.^[60] With more French being spoken, societal progress, culture, and universalism gave it an aura of "respectibility".^[55] In contrast, Dutch garnered little consideration and was deemed a language for peasants, farmers, and poor workers.^[62] In addition to the geographical language border between Flanders and Wallonia, there was in fact also a social language border between the Dutch-speakers and French-speakers.^[42][59][63] French was the language of politics and economics and a symbol of upward social mobility.^[53] French poet Charles Baudelaire, during his short stay in Brussels, complained of the contemporary bourgeoisie's hypocrisy:^[64]