It (Leuven) has always been a Dutch speaking town. Well, the majority of the population that is. The administration (city, university) started using French at some point (19th century - mid 20th century).
clarification: used french in the 19th-century until mid 20th century.
Anyway, the fact that the university was still giving some courses in French in 1968 in what was officially a Flemish University caused the "Leuven Vlaams" student riots.
I live in Leuven and you can clearly see that in the middle ages, they spoke dutch here. The switch to French and back is visible in the street names (because often they carry all 3 names). A funny example: The "grasmusstraat" (now) was "Rue Erasmus" in the 19th century and "grasmusstroike" in the middle ges. Apparently the French civil servants didn't know that a grasmus is a little bird and renamed it after a famous alumnus.
