In Europe is common for elevator to use zero for ground, even if language/culture has a specific name for it (piano terra/piano rialzato here)

Actually, "rés-do-chão" would be the more common term. Still... in Portugal, the "first floor" is implicitly never the ground floor (but the floor above it), so 0-based indexing is respected (as you suggest).

On elevators you don’t see “R/C” unless the elevator is very old, and the same for office building directories or shopping centres. In almost all cases it’s the number zero. OTOH, in countries like Germany you will see the “E” for Erdgeschoss everywhere.