I did a quick search, it seems it's similar to imperial and metric in that it's only the US doing 1st floor as ground floor. It's for various reasons, but in most European languages the word used for the numbered "floors" either means "horizontal division between floors" or the first "construction over the previous floor", so it makes sense that the first is the first above the ground.
It's like the basement, the ground floor is special.
And it's numbered different building to building, sometimes level 1 is nearest to surface, sometimes it's the deepest one.
And if you think that's confusing, I've ridden this one elevator once, it had four buttons arranged in a square: "P", "FSZT", "MFSZT", "1E". Guess what order the floors are in.