The country/language settings of the server or the personal setting are usually used to localize the website and the apps. This is not an individual rule set of Nextcloud but based on the world-wide standards.
So, if you, e.g. would use “English (USA)” a 12-hour clock is shown with AM/PM and the first day of a week is Sunday. If you would use “German (Germany)” a 24.hour clock is shown without AM/PM and the first day of the week is Monday.
So please be more precise and describe how you’ve set-up your profile and how your server settings are looking like.