in Sub-Saharan Africa many countries speak the language of their colonizer; French, Portuguese etc, yet the Congo is a Francophone nation, seeing as they were colonized by Belgium, why don't the people speak both French and Dutch?
In short: French was the language of administration, law, and education around the time of the colonization of the Congo.
When Belgium was absorbed into the French First Republic the French language was made the universal language. Administrative documents and education were both handled in French.
Following the creation of the United Kingdom of the Netherlands in 1815, the Dutch King William I reversed this, with Dutch being the main language instead. This attempt at enforcing Dutch failed for the most part.
With the Belgian Independence proclaimed in 1830, French once again regained its position as administrative language.
Some ten years after Belgian Independence, Dutch speakers became more consciousness of the language divide, and campaigned for equal treatment.
In 1873 law becomes bilingual, in 1878 administration, in 1883 education, and in 1898, both languages are made equal as official languages.
Despite de jure equality, French remained the language of the elite. Even as late as 1940 it wasn't uncommon for officers in Flemish units to only speak French, for example.
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