Issue Summary

The start day in the Create Event popup menu (in a Calendar) does not rely on:

• Global Language Setting
• User Profile Setting (Language)
• Team Calendar Configuration 

The issue is not reproducible in Confluence prior to version 8.8.0. Versions tested: 

Steps to Reproduce

1. Create a vanilla Confluence instance based on en_UK language
2. Make sure that the start day in the calendar is set to Monday
3. Change the language in user profile to English(US)
4. Check the start day 

Expected Results

The start day for the calendar, date picker macro in the page and the date picker in Create Event popup menu should be set to Sunday

Actual Results

The start day is set to Sunday for the calendar and the date picker macro in the page. 

However, the start day of the date picker in the Create Event popup menu remained Monday

Workaround

The native date-picker cannot be configured in Confluence, but the user can configure it using the operating system or the browser configuration to select the language for the weekday names and the day the week starts. Chrome and Safari will use the OS locale configuration, while Firefox will use its language configuration.