I just started translating Caesar's Commentaries on the Gallic Wars for my Latin class and I was wondering what would be proper demonyms for the peoples described in the book. For example, there is a tribe called the Helvetii in Latin. I assume it would be correct to call the Britanni Britons instead of British or using the Latin name, but it begs the question, what do we call a people who were around before the English language existed and how do we come up with this name? Should I call something like a river by its French or Latin name? Sorry if my question is confusing, but I'd be interested to learn more about demonyms and geographical names in historical context.
it begs the question
It doesn't, actually. "Begging the question" is a logical fallacy, it doesn't mean the same thing as asking or raising a question.
The translation of ethnic terms, place names, and the like is one of those things that varies from place to place, person to person, and time to time. A hundred years ago if you asked this question in the English-speaking world many people would have told you to substitute original place names at least with more familiar modern ones when possible, at least in publications that were intended to be broadly circulated. Thus we see Robert Graves (a classicist by training, a poet by inclination, and a wacko by PTSD) using France and French for Gaul and Gauls in I, Claudius. A contemporary of Graves', the great classicist Ronald Syme, used Latin terms instead, including for personal names (e.g. Antonius for Antony, both of which you'll find in English publications). There's not much of a method to it, it depends a bit on personal taste, the requirements (if any) from the publisher or department, and to a certain extent style. It's become common, for example, to attempt more "Greek-looking" transliterations for Greek words. Thus, Phokia for Phocia. But then again literally nobody in the English-speaking world writes Athenai or Athenae for Athens. Woe betide anyone who mixes these sorts of transliterations with Latinizing ones! In academic writing it's relatively rare to see Anglicized demonyms and place names, except in a few cases like Britons (usually where the name is so well known as to be obvious). Personal names in English publications may or may not be Anglicized, within reasonable limits--Pompey is common, but Tully is not anymore. Increasingly it appears to be common to eschew all Anglicization whatsoever in academic writing, in which all these words are familiar, but the point is always one of clarity. If nobody understands what you're talking about that's not ok.
I believe there is the word "Helvetian"