Well, that's usually a more complicated discussion. In French and Romanian at least, the word used for "floor" in this context ("etage"/"etaj") refers to an elevated construction. So, a building with "1 floor" ("1 etage") has 1 elevated construction atop the ground. This is what leads to the apparent 0-based numbering. The numbering is actually 1-based, but people count the extra layers above the ground, instead of literally counting the floors. In fact, I have rarely seen elevators show a 0 - they normally have something like P, 1, 2, 3.

It's more complicated anyway, since there are many structures with intermediate floors below or slightly below the ground, and those are usually nominated specifically ("mezzanine", "demisol" , etc.).