Here’s a question for you; what if an 18-year-old American citizen, property owner, voter, and registrant with the selective service left the United States and joined the French foreign Legion? Would he, at the end of his enlistment, be only a French citizen an American citizen or both a French and American citizen (dual citizen)?
Probably dual citizenship, unless France gets into a rumble with USA while you are serving in the FFL, particularly as an NCO. Which, despite our disdain for each other, isn’t particularly likely.
Possibly France has a requirement that foreign citizenship be renounced in order to become a French citizen. Don’t know about that.