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.

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. The fact that the Brig obviously does not understand Hokkien in this instance is read by some fans as implying that he hasn't been given what the Fourth Doctor called the "gift of the TARDIS" (TV: The Masque of Mandragora) and is therefore not truly a companion.