I tried to change the form properties-->Defaults--> Form locale also the Locale of Date/Time field object.
Please note that this has been tested on French desktop.
I may assist in french if you wish, is the software locale in french or english?
As far as I'm aware, the date picker is based on the software.
The date picker is software embed, different for Adobe Reader, Foxit or Power PDF.
It should change based on the selected software language.