Not sure what that has to do with calendars. Fortnight is just one of many measurements that aren't used a lot in the US (among other places) these days so I wouldn't expect the average person on the street to immediately recognize what it means. There are a ton of imperial measurements that aren't widely used like pecks, bushels, rods, etc.

(I'd also avoid terminology like bi-monthly and semi-monthly as it's a predictable point of confusion.)

Everyone in Britain knows what a fortnight is, and the word is in normal use.

We are confused when TSA staff in the US talk about quart bags.

It’s just not a word in everyday speech.

A quart is the easy one. It’s just a slightly smaller version of a liter. And it’s not like the UK doesn’t use pints which are just half of a slightly larger quart.

I’m sure there’s lots of language including unrelated to measurement which differs across the Anglosphere.