Even in a country as small as Italy it is very easy to recognize whether a sentence, even if pronounced and written with precise grammar, is from the north or the south of the country. It is all a matter of using certain words rather than others, slightly different lexical and grammatical forms, even the inflection with which they would be read. A foreigner probably wouldn't even realize it, but an Italian notices it instantly.
So let alone if such a difference in speech cannot exist in a country the size of China...