Latin

Latin was an ancient Earth language, spoken in Rome and its empire. Although the Doctor frequently used Latin expressions, many of his 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)
 * Although not noted in the serial, "magna peloris" simply means "giant clam".

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)

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)

Latin words phrases that the Doctor and his friends have used included caveat emptor, dona nobis pacem, magister, morituri te salutant, solvitur ambulando, status quo and veni, vidi, vici. (TV: The Fires of Pompeii, The Poison Sky, The Dæmons, The Masque of Mandragora)

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) Similarily, Winston Churchill was heard to be speaking Celtic by a Roman soldier in ancient Britain when he addressed him in Latin. (AUDIO: Living History)

Father Angelo was fluent in Latin. (TV: Tooth and Claw)

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 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)