activeindolence You mean above, mais c'est pas grave 😁
When I run LANG=fr_FR.UTF-8 thunderbird
from a terminal, the mail client and the calendar are both in french.
As Josh already said,
In Preferences -> General -> Language, the value should be French.
You can also check Menu -> Add-ons -> Languages and check if French is enabled.
I noticed the toggle to enable/disable Add-ons is only available on the extensions now.