Latin

Latin was an ancient Earth language, spoken in Rome and its empire.

Although the Doctor frequently used Latin expressions, many of their companions — such as Sarah Jane Smith and Donna Noble — did not speak a word. (TV: The Masque of Mandragora, Terror of the Zygons, The Fires of Pompeii, The Poison Sky) Other companions such as Harry Sullivan did, however. Harry once said "Magna peloris" to the Fourth Doctor after being saved from a giant clam; the Doctor did not understand him. (TV: Genesis of the Daleks) One of the Fifth Doctor's companions, Marc, was from the Roman Republic, and so spoke Latin fluently. He could also read in the language. (AUDIO: Interstitial)

When the Tenth Doctor and English-speaking Donna Noble arrived in Pompeii, Donna wondered what would happen if she spoke Latin to the locals, whose native language was Latin. Instead of hearing Latin, because of the TARDIS translation circuits, at least one native speaker of Latin believed her to be Celtic. (TV: The Fires of Pompeii) Similarly, Winston Churchill was heard to be speaking Celtic by a Roman soldier in ancient Britain when he addressed him in Latin. (AUDIO: Living History)

According to her servant Richard Maynard, Lady Peinforte "had some Latin". (TV: Silver Nemesis) Lemuel Gulliver also claimed to speak the language. (TV: The Mind Robber) Father Angelo was fluent in Latin. (TV: Tooth and Claw)

Examples
Both "Magister" (TV: The Dæmons) and "Dominus" were said to be the Latin word for "Master". (AUDIO: Vampire of the Mind)

Many plants were given Latin names as their scientific designations: herbabaculum vitae was commonly referred to as the "weed plant," (TV: Revelation of the Daleks) and solanum tuberosum was potato. (PROSE: Alit in Underland)

The word medicine was derived from the Latin word for healing. (AUDIO: The Sleeping Blood)

History
In 1400, Latin was used by the English clergy. The Lollards felt this was restrictive as it ostracised commoners who were not Latin-literate. (AUDIO: The Doctor's Tale)

Latin was taught in schools; Hutchinson at least once made Tim Latimer do his Latin translation homework at Farringham School for Boys. (TV: Human Nature) When Susan Foreman was a student at Coal Hill School, she confused Latin and French. (PROSE: Time and Relative)

When the Cardiff Space-Time Rift fractured, mixing space and time periods, a man Andy Davidson identified in the 2000s as dressed as a Roman soldier and not speaking a word of English was revealed by Jack Harkness to be an actual Roman soldier who was shouting in Latin. When Andy said the only word he could pick out was Gelligaer, Gwen Cooper noted that there was a Roman fort out at Gelligaer built around 75 AD. Jack suggested that the soldier was on his way there and when time splintered he ended up in their present. (TV: End of Days)

When the Fifth Doctor brought Marcipor to the 30th century from 1st century BC Cumae, the TARDIS translation circuit allowed Marc to see Nyssa's handwritten note to appear as Latin to him. (AUDIO: Interstitial) Back in Cumae, he had to hide that he could read Latin from his master Cicero, as slaves were not meant to know how to read. (AUDIO: Tartarus)

Although it was Earth's twin planet, Mondas had no Latin language. (PROSE: Alit in Underland)