The people of Flanders speak Flemish, not French. Clue is in the name 'Vlaanderen', vlaams being the Flemish name for Flemish!!
The Dutch Resistance members were most likely able to identify 'Bob' as a German spy not only from his too-perfect answers, but also how he wrote the date as DD/MM/YYYY. American soldiers in WWII were taught to write dates as MM/DD/YYYY.