[MITSardine]

Hmm, "en 8" makes sense to me in that you're using it to reference the next Whateverday that is at least 8 days apart from now.

If we're on a Tuesday, and I say we're meeting Wednesday in eight, that Wednesday is indeed 8 days away.

Now I'm fascinated by this explanation, which covers the use of 15 as well. I'd always thought of it as an approximation for a half month, which is roughly 15 days, but also two weeks.

To partially answer the other Latin languages, Portuguese also uses "quinze dias" (fifteen days) to mean two weeks. But I don't think there is an equivalent of the "en huit". We'd use "na quarta-feira seguinte" which is equivalent to "le mercredi suivant".