Aparently firstDayOfWeek attribute works as expected until dateTimePicker of the schedule header is clicked. It doesn't open the datePicker panel and all the page freezes.
For reproducing the issue it is only needed :
Tested on Angular 7.2.7 with Angular CLI
In the other hand, the documentation says that when setting a locale it takes the first day of week of the locale, but this doesn't happen. If you set locale property of the schedule to "es" (spanish) it translates the text well, but doesn't change the first day of week to Monday.