I was taught that the upper classes of present day Belgium, both noble and non-noble, started speaking French around the time of the French revolution and subsequent conquest by Napoleon. It even replaced the Walloon language entirely, even after the country became independent. Why and how exactly did it happen? Did the French oust the Walloon and Flemish speaking nobility and replace them with French ones when they took over the territory? Why did the same thing not happen in the Netherlands proper, which was also ruled by Napoleon at some point? Why did the upper classes not revert to their original language when Napoleon was defeated? Why did no-one adopt the German or Spanish languages when those empires held the Southern Netherlands?
This question interests me because it lies at the root of Belgium's ethnic/language friction up until the present day. I never quite understood why French was the language that stuck around, instead of the languages of Belgium's other historic rulers.
This is a much deeper question than it first appears - it gets to the heart of the state-building projects undertaken during the 19th century, and how it changed Europe.
It was not unusual for the areas that we know associate as a modern country in Europe to speak a wide variety of languages even up to the mid to late 1800s. Claims vary, but a minority of people in France (probably around 25%) by even the mid 1800s actually spoke France. Part of building the modern state, and developing economically, was getting everyone to speak the same language. Part of this was accomplished by coercion (forcing schools to teach in a national language, as opposed to, say Flemish or Walloon dialects). However, some of the pressure was internal - in order to interact with the powerful French government and French economy (which was very important, especially in southern Belgium), one had to learn French. This was despite these areas being beyond French national boundaries. So while Napoleon helped inaugurate the era of state building, it was the fact that the expanding French state only really mattered enough economically to force the adoption of a new language. Of course the people that felt this force the most were the wealthy - they had the resources to most easily take part in the growing French economy. Those further north, who were further from the Isle-de-France, were less likely to adopt French.
Much of this info comes from Weber's Peasants into Frenchmen, which I conveniently was reading right before coming onto reddit to procrastinate.
This answer is from Memory and it's how I learned it in my History classes (20 years ago).
/u/Clockt0wer 's answer is probably more accurate but I want to share the way I learned it.
Dutch as spoken in Flanders is a Germanic language and the "Walloon" language was a Latin language, just like french and it indeed was very similar to french I think it was even (wrongfully?) called a french dialect.
In times of Roman occupation on the south side of the language barrier there was a higher density in roman settlements and Romans. On the north side there were Roman settlements but there was a lower density. So Latin was never really common in the north of Belgium.
The fact that the south already spoke a Latin language so similar to french would probably have contributed to the Walloons having picked up the french language for the reasons explained in Clocktowers post.