Well, this is very funny :)
I have a datepicker localized in Italian. It works correctly, except for one small glitch:
Apparently, the abbreviation for Wednesday, was translated as a word, since "noi" is the Italian translation for the pronoun "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
dependencies: - core/jquery - core/jquery.once - core/jquery.ui.datepicker - core/drupal.locale.datepicker
and without the last line the datepicker is in English.
Am I doing something wrong?...