Hi Werner,
It's working for me. Maybe it has already been fixed for next release, or maybe the users that you tested already had chosen a value for those user config options. Note that default user preferences only apply if you haven't chosen a value before. This could have happened if:
1) At the beginning you had "Show week numbers" disabled and "Start on monday" disabled.
2) You enabled both of them at some point and disabled them back.
3) Now you changed the default value for those options, but since you had already chosen a value for those options they stick with the values that you had chosen before.
If a user never changed the value before then changing the default value should affect the user.
Please try if it is working for new users (if new users' calendars start on monday)
Thanks.