The answer is: it depends on the country and culture. In Canada (both French and English, except in hogarth’s cubicle) we use the same convention as in the U.S.: Sunday at the left. In many countries in Europe, it’s Monday at the left.
If you publish a calendar that clashes with the local convention, people may choose not to use your calendar. Or worse, they may rely on your calendar without noticing it doesn’t match their other calendars, and get a “Darn foreigners!” reaction when they miss an appointment because of it. So you should find out which convention is most common in the target countries.
Windows Regional Settings (and probably MacOS too) lets you choose which day is leftmost.
Your Outlook calendar is Monday to Sunday because someone set it that way (go to options –>calendar). When I worked 4 days a week instead of 5 mine was set Tuesday to Monday.
Anyway, I wouldn’t buy a wall calendar that began the weeks on Mondays. I’ve had appointment books like that and really haven’t liked it.
Every university / college I’ve ever attended (three so far) has had Mon-Sun weeks – for example, next week, starting Monday, is week 8 of the semester. Today and tomorrow are in week 7.
My retail job, which I’ve had for almost 15 years, uses a Sat-Fri calendar for payroll and scheduling, Sun-Sat for accounting, and Thu-Wed for the weekly ad.
My brother’s old jobs in restaurants were either Wed-Tue or Thu-Wed for scheduling and payroll.
I actually prefer Sat-Fri, cause that’s what I’ve based my life on for forever, but I’m adjusting to Mon-Sun.
I accidentally bought a Monday-Sunday calendar for work one year and my boss threw it away after one month because it was too darned confusing. Our Pay/Schedule/Goals week runs Sun-SaT and every calendar I’ve ever owned (for longer than a month, see above) has been Sun-Sat.