Mandarin

Mandarin was a dialect of Chinese. In the early 21st century, it was the most prominent language on Earth. About a billion people on Earth spoke Mandarin, three times more than the English language. (TV: Children of Earth: Day One)

The Fourth Doctor stated that he could speak Mandarian, among other Chinese languages. (TV: The Talons of Weng-Chiang)

Rex Matheson ran some footage through Central Intelligence Agency software to translate it from Mandarin to English. (TV: The Gathering)