Here's why:
* French was the language of the ruling class in Saint-Domingue (Haiti). Toussaint was born into slavery and likely learned French through his interactions with his masters, as well as from the French education system that existed for some slaves.
* He used his language skills for communication and political maneuvering. Toussaint was a skilled negotiator and strategist who often wrote letters and documents in French.
* He fought for the abolition of slavery and the independence of Haiti. This would have required him to communicate with other revolutionaries, officials, and foreign powers, all of whom would have spoken French.
While he likely had some exposure to other languages like Creole, French was the language of power and literacy in his time.