Mandarin is the standard spoken language (dialect) in today's China, meaning distinct regions may have their own dialect, but they can all use Mandarin to communicate with each other.
Since all languages evolve, as a contemporary Cantonese speaker, I would have difficulty understanding Cantonese conversations from just two hundred years ago. How would a conversation between Cao Cao and Liu Bei have sounded like? What standard dialect would they have been using?
Edit: This question is about the history of "standard spoken language" which refers to standardized languages used for official purposes in China. This question is not about the history of many vernacular languages spoke throughout history in China.