The abbey of Leffe is located in the French-speaking part of Belgium. French pronunciation is /lef/. Dutch pronunciation (with AB-Inbev headquarters located in Flanders) in Belgium is /lef-ah/. But apparently in the Netherlands they (not sure whether it's everybody, maybe just beer snobs) pronounce the name the French way, /lef/.
Completely logical, no?
This having been said, I never drink it, and I never pronounce its name except when I talk to foreigners in Belgium who, for some reason I don't understand, think that Leffe is the creme de la creme of Belgian beers. It isn't.