In the properties of the calendar widget there is an option "Today button text". This is nice for translations but not available for the buttons week, month and day. The documentation suggests this should be possible. (http://arshaw.com/fullcalendar/docs/text/buttonText/)
I'd have to check but if I remember correctly, these are simply the default values when no alternatives are set.
If you re-create these buttons in the Modeler with Dutch names, they should replace these.