This article, "A Brief History of Dutch in Africa", explains:
From the start, Stanley worked with officers and agents of a variety of nationalities, many of them Belgians. These Belgian nationals were of either Dutch-speaking or French-speaking origin. As at that time French was still the only language for all formal communication in Belgium, the Belgian officers and agents in the Congo quite naturally used French as the official language among them and for writing. [...] In sum, the arrival of Flemings in the Congo in the late 1870 and 1880s, marked the beginning of a structural presence of Dutch in Central Africa, albeit always under the hegemony of French as official language.