Caratacus was a British chieften who famously gave a speech to the Roman senate after having been defeated and captured. Given Britain's great distance from Rome and it's relative isolation, how was he able to give such a speech in any kind of way that was properly understood?

Moreover, how did the Roman legions communicate with the locals when so far afield? I can understand neighboring and conquered peoples having translators with skill in both Latin and their local language. But it seems unlikely (given the little I know) that places as far flung as Armenia, Parthia, and Britain would have such translators (especially in the time of Gaius Julius Caesar when he went to Britain). My impression of the Romans around that time is they had a strict adherence to legalism and procedure with adversaries both before and after defeating, so being able to communicate properly must have been essential.