Hi,
Jquery ui datepicker is translated in wordpress since the version 4.6 : https://make.wordpress.org/core/2016/07/06/jquery-ui-datepicker-localization-in-4-6/
Instead to ask each user to translate manually the datepicker (https://docs.gravityforms.com/translating-the-datepicker/) Gravity Forms should automatically translate the datepicker itself.
Thanks