In the United States we use scientific pitch notation to identify each note on the piano keyboard. Each note is identified by its letter name and its octave number. So in SPN, Middle C would be C4, and an octave below that would be C3. The lowest note on the piano is A0, and the highest is C8.
I know that many non-English-speaking countries use solfege (i.e. do-re-mi instead of c-d-e) and I've heard that in China they simply use the numbers 1-7. Also, I know that other pitch notation standards exist, like Helmholtz pitch notation where middle C is c'. But it is not clear who uses what notation. For instance, in Europe, would middle C (C4) be do4, do3, do', or something else? What about in Asia?