add a monthNames option after the dateFormat setting. fro example:
$(function(){
$('#datepicker_1').datepicker({
dateFormat: 'mm/dd/yy',
monthNames: ['Januar','Februar','Marts','April','Maj','Juni','Juli','August','September','Oktober','November','December'],
showAnim: 'show',
onClose: closeDatePicker_datepicker_1
});
});