MacOS – Pages 5.2: How to insert date field in other language than system default

To follow up Martin's answer, for British English you will need to edit the list and check the "British English" tick box. I spent while looking for "English (British)", but it's the former format that appears.

change the region under System Preferences > International > Formats. I believe that most of the region settings will cause Pages to default to ISO paper sizes (such as A4). The only ones I know of which result in US Letter are United States, United States (Computer), and Canada.

If all you care about is the paper size for new blank documents, you can always save a custom template with your preferred page setup, use that, and not touch your system settings.