7.2.X, 7.3.X, Master
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 Dynamic Data Lists and create a new data definition with a Date field.
4. Create a list based on this definition and start adding a new record to it.
5. Click on the date field.
Result: Sunday is the first day.
Expected: Monday should be the first day. (If at step 2 you would have set user's language to French, you would have Monday-Lundi as the first day, correctly)