There’s a lot of horse trading goes on with standards and really the ISO just hoists various national standards that begin to be adopted by other nations into international ones. The only American one that I can think of are the old camera film speeds which used to be ASA100, etc. and then became ISO/ASA100.
Don’t know why but TR have made a bit of a mess of time and date - they’ve fixed on the American date format (MM-DD-YYYY) for example rather than allowing users to specify the one used locally (YYYY-MM-DD or DD-MM-YYYY). You should separate the underlying value from the format displayed to the user.
Self-selected start of week shouldn’t be a problem if the underlying framework is set up correctly. In theory you could display “weeks” of 7, 8 or whatever number of days, again it’s a separation of underlying data from how it’s displayed.