In Kant's day, French was essential for any academic and educator, even if just a tutor. French was the language of leading science and practically the official language of the Enlightenment.
Kant recognised this and was quite proficient. However, nothing can substitute practicing with native speakers. Kant had a great deal of it while working for Pastor Andersch in the town of Jüdschen, where the majority of the population was made up of religious immigrants whose native and sometimes sole language was French.