It is really tricky to explain and even trickier to prove to readers who have perhaps not so much experience with different languages that just because a word X in foreign language is translated to word Y in English does not mean they represent precisely the same concept. 1/8
Yeah, I think metropole is one of those words you want to leave untranslated. Certainly I think most scholars do when discussing imperialism or France (or French imperialism), treating it as a technical loan-word for a particular idea.