evant, thanks for the quick reply.
The problem is with pre-setting a value for startDay, not setting it on a one-by-one basis. The german locale that the original poster referred to sets Ext.picker.Date.startDay to 1 which stays without effect. I worked out in the meantime that I need to set startDay for Ext.picker.Date and Ext.form.field.Date separately. I would expect the datefield to use the same setting as the datepicker by default. Otherwise the german locale (and possibly others) should include the setting for datefield.