How can you forget? Growing up in Australia, every computer I’ve ever owned has defaulted to “US Letter” as the paper size. (Even when your computer was set to use Australian localisations.) If you didn’t change the setting, you couldn’t print. The printer would complain that it was out of paper, because it didn’t have a tray with the requested us letter paper in it. It was one of those things you always had to check.

I don’t think I saw a “us letter” sized piece of paper until I was an adult.

Every windows install I've ever done (in Australia) I picked "English (Australia)" when possible, and "English (UK)" before Microsoft provided that localisation option. I've never seen US letter as a default.

That's funny because (living in the US) I've noticed that many printers default to A4. Maybe 20 years ago, one printer at work would think A4 paper was loaded in it after every power-failure and we'd have to go through the menu and change it, or it would require you to clear a warning before it would print anything sized for US letter.

Must be an Anglosphere thing. Every non-English computer and office software I've ever used correctly defaulted to A4.