It's pretty much international amongst most western civilised countires/nations, and some eastern ones too. Whilst I don't know the exact time the prank started here, I was aware of it at approximately 23:30 UK time on the 31st of March.
But hey, it's been over for almost a couple of days now, so let's not keep on about it...
Yep, now it is widely known, but it is still very different the length to which they go in anglo-saxon (mostly USA) countries to do it, and other countries, where most people make jokes about the 1st of April, but do not actually organise complex things just for that (and were newspapers or news agencies do not publish fake news for the 1st of April).
Effectively, some people try to link it to even to some festivity in ancient Rome (but in reality, that festivity was linked to a specific cult in a specific historical period, and it was not really like "April's fool", but rather about simply mocking people directly, more akin to irony, same as the celebration inverting master and slave, although this second one was not only specific to a cult, and is documented over a long period of Ancient Roman history), and they link it also to the change of calendar in France in the sixteenth century (as a single episode), but even those admit it is in reality in UK in the 18th century that the "April's fool" started being diffused and systematic, and from there to USA.
BTW, I know it is a bit off-topic from the game, but if you look, Myriam gave also rise to conversations about different legal systems and laws (though about subjects somehow linked to the game, like incest law, rape law, right of non-married legal couples, etc.), and I think those are nice, a way in which Myriam helps us all to expand ourselves (I mean mentally, not in other areas
;-)).
Maybe Zorlun can even include the idea of someone trying an "April's fool" on Myriam in the next update
;-)