There were a mix of different forces that caused French to be the colonial legacy language of the Congo, but the chief force was the desire of local elites to the social status and opportunities that French offered.

Most education in Leopold's Congo Free State and then the Belgian Congo was undertaken by state supported missionary groups. Catholic & Protestant groups received backing to essentially undertake the 'civilizing mission' that the Belgian state was otherwise ambivalent towards. Ambivalent in terms of policy on the ground, but as Roland de Marès revealed in his full throated defense of King Leopold in 1904, in terms of international publicity the civilizing mission took center stage. De Marès argued that those who were critical of the Belgian efforts in the Congo were jealous of Belgium's success and adherence to the duty of it's "high mission of progress and civilization." The thing is, those missionaries who led education efforts in the Congo, were typically uninterested in 'westernizing' indigenous societies as much as they were interested in 'christianizing' them.

Teaching indigenous peoples French or Dutch was not seen as helpful in saving souls, better that the missionaries learn local languages (or trade languages) and translate Christian material. Or, teach people Latin. Aside from a preference for non-living European languages, a large portion of the missionaries were themselves anglophones or dutch speakers and not native born French speakers. There was little support for French from the missionaries who feared creating “harmful and dangerous underclasses.” Barbara Yates argues that the only reason that English did not become the lingua-franca of the colony was the threat of closer state supervision over missionary curriculum during the 1890s. Leopold's administration feared that the spread of English might facilitate an eventual English takeover of the Congo and so it sought to halt its diffusion.

There was a fear that with the French language came a set of ideas, in fact only a few French words were believed enough to give the African ideas about assimilation and civilization and the more Africans who learned French the greater the threat to colonial control would be. There was also a fear that French could become a unifying language that allowed a challenge to white hegemony. And indeed Congolese thinkers in the 1950’s presented this very idea that French could be their unifying language, that it was the civilized language of communication and brotherhood and it could be used to challenge the White/Belgian authority.

The reality is, that French was the privileged language of Belgium-- it was effectively the language of the elite and the state in the colonial setting. So when indigenous groups wanted to interact with the state on equal footing-- or wanted access to the social mobility that working for the colonial state-- then French was the language you needed to learn. Preference for French was picked up on by the missionaries who altered their approach to offer French language education, after all, if a rival missionary group is going to offer French and accede to indigenous requests, then you had better also begin offering French if you can or risk losing potential converts.

Some reading that might be of interest to the subject :

• Meeuwis, Michael. “Bilingual inequality: Linguistic rights and disenfranchisement in late Belgian colonization.” Journal of Pragmatics 43 (2011): 1279-1287.

• De Marès, Roland. The Congo. Brussels: J. Lebègue and Co., 1904.

• Fabian, Johannes. Language and Colonial Power: The Appropriation of Swahili in the Former Belgian Congo 1880-1938. Great Britain: Cambridge U Press, 1986.

• Yates, Barbara A. “Educating Congolese Abroad: An Historical Note on African Elites.” The International Journal of African Historical Studies 14.1 (1981): 34-64.

• Yates, Barbara A. “Shifting Goals of Industrial Education in the Congo.” African Studies Review 21.1 (1978): 33-48.