This LPS springs from https://issues.liferay.com/browse/LPP-41471, see the history there.
Steps to reproduce:
1. In Instance Settings/Localization add English (UK) as a current locale.
2. Set the current user's profile language to English (UK) - in Account Settings. This means that whenever this user is presented with a date picker, he should see Monday as the first day of the week (see https://en.wikipedia.org/wiki/Week and
3. Go to Web Content and create a web content structure with a Date field (and a dummy template for it).
4. Start adding a new web content article of this structure, click in the date field.
Result: Sunday is the first day.
Expected: Monday should be the first day.
(Note that at the right side, if you go to the Schedule section and click on the Display Date field, you see Monday as the first day, correctly.)
Analysis of TSE on LPP-41471:
the Date field in Web Contents are insensitive to localization setting. Not even the name of the days are translated
Date field should be made aware of the locale.