So, in The Mind of Evil
The Doctor not only demonstrates his knowledge of Chinese but also claims that he knows Mao personally and that Mao offered him to call him Zedong, a fact the Doctor seems to be proud of.
Obviously, this also raises some ethical questions but mostly I started wondering when in Mao's life the Doctor met him and which Doctor it was. Was that ever expanded upon in the show or in tie-in fiction? Also, what are your thoughts on the matter?
As it is the Third Doctor saying these things while he's still in exile on Earth, it must have been one of his predecessors. My money is on the First Doctor sometime before we meet him. In the early serials he seems to be less concerned about objectionable behaviour in the ethical department and he's on record saying the French revolution era is his favourite. So, a visit to revolutionary China sounds like something he could have done.