In The Annals, the Roman historian Cornelius Tacitus wrote about the British chieftain Caractacus and how he was captured by the Romans. According to him, Caractacus was taken to Rome along with his family to be publicly humiliated and then executed. But, before his execution, he managed to deliver a speech to the emperor Claudius. After hearing the speech, the emperor decided to pardon Caractacus and set him free.
We don't know if the speech was so eloquent as Tacitus portrays it, but it was probably not so inarticulate, either. Besides, the emperor (and Tacitus himself) seemed to understand it very well. The problem is: Caractacus's native language was Brythonic and the Romans, of course, spoke Latin. So, in what language did he deliver that speech? Is it possible that Caractacus was fluent in Latin? Or, what seems more plausible, were Brythonic and Latin mutually intelligible (cf. Italo-Celtic hypothesis)?