During pairing in English, you have just to press "yes" at this stage !

And in French, the "Search again" is far too long and so "Oui" = "Yes" is not visible !

Stupid scaling problem but really a hugly one. So if it can help somebody, hope that you will find this post :p
So workaround, switch to English language, pair your Fenix 3 and go back to French and pairing is working :cool:
For Garmin developers, manage button size or just translate "Search again" by a shorter one like "Rechercher" !
---- French version - version française ----
Si vous avez un soucis de connexion bluetooth dans Garmin Connect Android, alors il suffit de basculer en Anglais pour faire la connexion puis repasser en Français.
En espérant que cela soit utile à d'autres et que vous trouviez ce post !