I wouldn't buy this product again. Too complicated to use!

The language of the device can be changed using the device’s built-in menu or through the app. The app itself doesn’t have a language setting, but appears to follow the phone’s language setting.
The app is free but does require an account to use history and saved presets. This apparently also allows monitoring temperatures from outside the home (a dubiously useful festure) if the device is connected to Wi-Fi.

I'm sorry. I can't answer anything. Gave it away and no feedback yet.

And yes, French language is supported in my unit.