Well, this is very funny :)
I have a datepicker localized in Italian. It works correctly, except that, apparently, the abbreviation for Wednesday was translated as it were a personal pronoun, since noi is the Italian translation for we.
The abbreviation for Wednesday is correctly set in core/assets/vendor/jquery.ui/ui/i18n/datepicker-it.js, but this file is not inserted in the HTML, so I suppose the translation is taken somewhere else.
In my libraries dependencies I have the following code.
dependencies: - core/jquery - core/jquery.once - core/jquery.ui.datepicker - core/drupal.locale.datepicker
Without the last line, the date picker is in English.
Am I doing something wrong?