If you’re interested in Delta, also set up an account now when you’re outside the USA and leave that non-US address in your profile. Non-US accounts are exempt from the Medallion-Qualifying Dollar requirement for elite status and since Delta mikes don’t expire, you can credit one or two flights there and have it set in case you later need it.

If you do decide to join the DL program, I'd join it now using your French address. If you use a US address, you're required to meet minimum $ spending requirements to earn status... but this doesn't apply if your address is outside of the US.