Datepciker headers are incorrect when using locale (en)

[JorneDelforge]

Describe the bug

When using the react date picker in 'en' locale, the headers are misaligned. Sunday is first day of the week, but the calendar dates are still assuming Monday as first day of the week.

Screenshot attached.

To Reproduce

Steps to reproduce the behavior:

1. Go to https://digipolisantwerp.github.io/antwerp-ui_react/#form ==> date picker component, make certain locale is set to 'en' in the example code
2. Open the date picker and compare it to you system calendar. Day of week does not match the actual dates. Example: 1 august 2021 is a Sunday, not a Saturday

Screenshots

OS (please complete the following information):

• Device: MacBook
• OS: OS X
• Browser Safari & Chrome

Additional context

Has been discussed with Joeri S. When setting locale to 'nl' the issue is fixed.

[jsebrech]

There is an additional issue. When starting a fresh project with the Digipolis Starter kit, and including the Datepicker component on a page, locale=nl does not work. This is related to moment/moment#4216

As a workaround, one can put this code at the top of index.js:
import 'moment/locale/nl';