Mandarin works fine in Big China. You get out into the overseas Chinese community and you’ve gotta’ have facility in Hokien, Cantonese, Shanhainaise, and so on.
I know that. It is the same here in Taiwan. Around Taipei, most people speak Mandarin.But you go to some of the southern parts and ( mostly with the older folks- my inlaws and such ) they speak mostly Taiwanese, even though Mandarin is the "official" language. You even have Hakka spoken in some parts.