DateField.getDatePicker().setStartDay(1); // set monday as the first day of week
has no effect in 3.0.1: sunday is still the first day in the date picker. I hope, this bug fix will solve the problem for Germany, but it still should be possible to set the first day of week manually.
Changes were made in this class about five weeks ago to correct the behavior of setStartDay (EXTGWT-3096) and were included in the 3.0.6 release that went out late last week. Can you verify that it is working correctly in the current support release?