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.
I've heard that it has the historical explanation that back in time, the ground floor was often literally the ground, so the first floor was actually the first floor. Don't know if that's correct, but I seem to remember having heard/read it somewhere.
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.
This was a building in Budapest, "P" stands for "pince", as in basement, "FSZT" is "földszint", literally "ground floor", "MFSZT" is "magasföldszint", "high ground floor" meaning mezzanine level, and "1E" is "1. emelet", "first elevation", so that was highest.
The quality of the elevator still made me think of taking the stairs though.
Fun fact, Hungarian is the only language I've heard of that uses Latin letters and also has multi-glyph letters as long as four glyphs, so "sz" is considered one letter like in Polish I think, but "ddzs" is also one letter.