Will a Nokia 2780 flip phone work in France?
The Nokia 2780 is an older model (introduced about November 2022) and only has 4G capability so it may have problems in some areas that now primarily use 5G. The best answer I can give is that it should work, assuming your cell provider allows roaming in Europe.
All of France has 4G and quite a bit has 5G in addition
3G works as well, but it will be decommissioned in 2030. 2G will probably be gone by the end of this year.
It's interesting to see a 2022 phone described as "older". Predictions are that the replacement cycle will become 3 years in France - currently 4. But they've been saying that the last 10 years. My replacement cycle is 7 years, and I'm guessing I'm typical of country France.