As an Aussie, I was amazed to learn that most Americans had never heard of fortnight (or maybe knew exactly what length of time it is). I guess the funny thing is that it is a non-obvious contraction of "fourteen nights" - and not "forty nights".
Worse, there was a period of time where Americans would shorten a phrase, then mis-spell it for the lulz. The most famous example is "All Complete" -> "Oll Komplete" -> "OK" -> okay.
Every British tabloid has a regular, very popular column peeving about yet another abomination by the Americans, or the South Africans, or the Australians. Without exception they note what is simply a local fashion lately abandoned locally, but preserved as if in amber elsewhere. The complaints are really about distant people failing to slavishly follow current London trends by abandoning usage London has abandoned.
The peculiar popularity of such peeving could be interpreted as wistfulness for a time when London mattered.
And, by the way, that derivation of "OK" is suspect. It is just as likely to have been (fashionably) sarcastic commentary on well-established common oral usage. Etymologists are generally obliged to rely on written sources, but word origins routinely predate the first written use by many decades. Slang originating in exclusive universities is vanishingly rare.
Fortnight derives from Old English "fēowertyne niht", meaning "fourteen nights"
Are you sure that was not the case for non native speaking folks in living in the UK.