Chinese is not easily comparable with European languages. Certainly, Cantonese has a very different pronunciation, grammar and lexis from High Chinese. But if, for example, a Beijinger can read and write traditional characters, he is able to communicate in writing(!) with a Hong Konger without any problems. In my experience, many Mainland Chinese can at least read the traditional characters.

The term “Mandarin” is (sadly) still used in English-speaking countries. What you mean are the few Northern dialects, what are not so different, refined with the 官語 (language of the civil servants) and a portion of the Beijing dialect.

Or, if you count the speakers worldwide, Chinese would be the choice … :D