Hokkien

Hokkien was a dialect of Chinese in which the Third Doctor once conversed with Fu Peng. Upon hearing the delegation chief's name, the Doctor immediately assumed that Fu would speak Hokkien. (TV: The Mind of Evil)

Behind the scenes
As compared with the year The Mind of Evil aired, Hokkien is now more associated with Taiwan than mainland China. Though not impossible for a Hokkien speaker to be representing the People's Republic, it does seem a bit odd to 21st century ears, given the still white-hot antipathy between the two Chinas. That said, Hokkien is prominent among Chinese abroad in Southeast Asia, and so there is some logic, even viewed with an early 1970s sensibility, to a diplomat being chosen from the Hokkien cultural group.

The reason that Hokkien was chosen was because writer Don Houghton had written the scenes entirely in English, and expected that the production crew would find translations by the day of recording. This effectively left translation up to his wife, Pik Sen Lim, who only spoke Hokkien. Leaving things so late disadvantaged Jon Pertwee, for whom the Hokkien had to be very much simplified. Even so, Pik said she can't completely understand what Pertwee was saying in his Hokkien-speaking scenes. (DOC: The Military Mind)

However, the "Hokkien mystery" went even further. The original actor chosen for Fu Peng was fired by director Timothy Combe after location filming but before these studio scenes. For the original actor, the Hokkien made some sense, but not for his replacement, who, according to Pik Sen Lim, spoke Cantonese. So neither Kristopher Kum nor Jon Pertwee spoke a language they actually knew. (DCOM: The Mind of Evil) The even deeper irony was that the firing meant that the two ethnically Chinese actors ended up having to learn lines in each other's native dialect — presumably because switching between Hokkien and Cantonese would have confused Pertwee a bit too much at that late stage.

Oddly, the Doctor seems to imply that he spoke Hokkien with Mao Tse-Tung, which may suggest a difference between the Doctor Who universe and the real world. In the real world, Chairman Mao was in no way a Hokkien speaker, but instead had a pronounced and quite obvious accent, since he came from rural.

Hokkien is one of the few real world languages to be subtitled in televised Doctor Who, since it is generally assumed that the TARDIS translation circuits are naturally giving everyone in any given scene mutual intelligibility.