Apparently the Old(e) English spelling for 14 was something like "fortene", so it made sense to them at the time. Guess they needed a handle for a time frame between a week and a month - "bi-monthly" probably didn't fly back then.
Neither did "semi-monthly."
Fortnight: Two weeks; you will hear this term in any country that has its roots in colonial english.
Yeah, around here you'd get the colonial Spanish version.