Exactly. The French word for "stem" is potence (po-tahns) which also translates as "gibbet" (the "7"-shaped frame they used to hang the bodies of executed criminals from.) But we don't call stems potences (or po-tahnses) or gibbets; we call them stems. Similarly a device that derails the chain from one cog to another we should call, in English, simply a derailer. I do, and so does every English-speaking Canadian I know, even if they speak good French also. But if you are ever looking for one in a bike shop in France, it will help if you can pronounce dérailleur. French has many sounds that simply don't occur in English -- to pronounce the words so as to be understood you have to make those unfamiliar sounds. It's not laying a French accent on thick, it's how the words really sound.