The Deep Time Reality (Geological History)
If you mean when was France naturally this hot as a baseline climate—where temperatures of 40°C+ and prolonged tropical nights were typical for the region—you have to travel back 5 to 15 million years ago to the Miocene epoch.
During the mid-Miocene, global CO_2 levels were comparable to or slightly higher than today, and Europe had a radically different climate:
The Landscape: What is now France was a lush, parched, subtropical-to-savanna environment.
The Wildlife: Direct ancestors of hippopotamuses, rhinos, and ancestral elephants roamed across Western Europe.
While individual, brief heat spikes likely occurred during the last interglacial warm period (the Eemian, about 125,000 years ago), a sustained regional climate where this level of heat blankets three-quarters of the country simultaneously hasn't been the "norm" for France since long before humans ever walked the European continent
