How change langage. Because i m frenchy
Where? On the phone, or on the watch (which model), or on another device, or on Garmin Connect web perhaps?
If on the Connect app (this is the forum for the GC app), then you do not change it. The language is automatically in the same language as your iPhone (see more details here: Changing the Language For Garmin Apps | Garmin Customer Support)
If on the watch, then it depends on the model, but typically it is in the watch menu » System » Language. If not your case, then consult the user manual for your model (Garmin Customer Support)
If on GC web, then the setting is at https://connect.garmin.com/modern/settings