Earlier quoted context omitted.
I came across this quote from the Mandarin wiki: > The Chinese have different languages in different provinces, to such an extent that they cannot understand each other.... [They] also have another language which is like a universal and common language; this is the official language of the mandarins and of the court; it is among them like Latin among ourselves.... Two of our fathers [Michele Ruggieri and Matteo Ricci…
I always find the claim that dialects in China vary so much that people cannot understand each other very western-centric. As a native speaker, there are shockingly few dialects that are completely incomprehensible. Someone from Shanghai will be able to speak to someone in Chengdu with their own dialect, just slower.
This is definitely not true (if they were to use their own dialect).
I'm from the same region surrounding Shanghai and although there are some intelligibility between dialects, it is hard if not completely impossible to have a conversation even between people from adjacent towns. See https://en.wikipedia.org/wiki/Wu_Chinese
I'm a bit surprised about your claim. Anyone from China with a sense of the dialects of the south would know this.