Date Picker does not work with recent XWiki version 12.8

I just noticed that when creating a new event or editing an existing one, the date picker does not show up in the modal dialog.
When editing an event on its separate page (i.e. by clicking on the "View" button in the modal of an existing event) the datepicker just shows up fine.

Likely this is caused by meddling with some internal stuff of the date picker, maybe when trying to load stuff when the model dialog opens.

The browser console shows the following stack traces:

Uncaught ReferenceError: assignment to undeclared variable _translations
    Externals calendarDateSelect.js:62
    <anonymous> calendarDateSelect.js:546

and then several of these:

Uncaught TypeError: Externals.CalendarDateSelect is not a constructor
    _trigger dateTimePicker.js:55
    bindAsEventListener prototype.js:419
    _createResponder prototype.js:7276