How to fully localize popup calendar field
Well I'm a bit ashamed :( No need to do such complicated things. In the edit field page, there are some advanced settings and values I had never paid attention to : 


Date entry options : control the order and format of the options users see.





Default date : Relative default value (Describe a time by reference to the current day, like '+90 days' (90 days from the day the field is created) or '+1 Saturday' (the next Saturday). See strtotime for more details.