Just to widen the confusion beyond the French and their language - in Spanish a two-week period is referred to as a quinceno, also based on the number fifteen. So, perhaps there is some divide among Romance and Germanic languages on this one. I can't recall if Italian has a similar construction. Can anyone contribute on other possible languages on this divide?

Another odd note - in Spanish, to say "a week from Thursday," one says "Jueves en ocho" which is basically "8 days from Thursday." Another illustration of this counting difference, perhaps?