Currently, labels for weekdays and months are not translated to the language of the Back Office, there is always only English translation shown.

Steps to reproduce:

1. Set the main language of the Back Office to Polish in the User Preferences.
2. Install the proper language package composer require ezplatform-i18n/ezplatform-i18n-pl_pl
3. Log in to the Back Office.
4. Open any datepicker, for instance, the one for the "Publish later" button.

Result:

Weekdays and months are translated in English instead of Polish, see datepicker.png.