Celtic language

Celtic was a language spoken by the Celt people of Great Britain. When the Doctor and English-speaker Donna arrived in Pompeii, Donna wondered what would happen if she spoke Latin to the locals, whose native language was Latin. Instead of hearing Latin, at least one native speaker of Latin believed her to be Celtic. (DW: The Fires of Pompeii)


 * What exactly happened is unclear. The TARDIS may have translated the Latin back into English, which the speakers thought was Celtic. The words may have not been translated at all, but Donna's pronunciation may have sounded foreign.