@frogworth @Hyperlynx @futzle I must be partially in some alternate universe.
Printed calendars here in Australia have almost always had Sunday as the first* day of the week as long as I can remember, and still do. And I seem to recall computers did it too for many decades, too. It seems something happened in the last 20 years that meant there was a gradual change to show Monday as the first day.
This may be what was mentioned with EU and ISO rules. I have changed multiple services over the years, an unknown number of which may have been recognising a local locale. I am not in a position to easily check some of the big players, but I did just checked the calendar add-on to the email service I use (Fastmail) and it defaulted to Monday. There was not an option to set my locale, but I could change it to Sunday. And I have already mentioned having to change Outlook on a work PC.
Curiously, the calendar in my browser (Vivaldi) which I also do not use, uses Sunday as the start of the week. And does not have an option I can find to change it!
This is all quite confusing.
* First day of the week and first day of the _working_ week are separate things! Maybe it's an unusually early type of 0-based counting?