I speak Cantonese and I’d argue that despite sharing a common written system, Cantonese and Mandarin are two separate languages, not dialects. Sure, if I listen carefully I can pick up a Mandarin word here and there but the two are basically mutually unintelligible. There are many, many variants of both languages throughout the region, and I’d say that those variants are the actual dialects.
From another perspective, many people tend to class Mandarin as a language and Cantonese as a dialect. Cantonese is actually far closer to Middle Chinese than Mandarin is – so perhaps it’s the other way around.
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