Ext.onReady(function(){
    if(Ext.MessageBox){
        Ext.MessageBox.buttonText = {
            ok : "OK",
            cancel : "Abbrechen",
            yes : "Ja",
            no : "Nein"
        };

        // ADDED THIS TO DIRECTLY ASSIGN THE TEXT TO THE BUTTONS, AS BUTTONTEXT DOESN'T WORK
        Ext.MessageBox.msgButtons['ok'].text = Ext.MessageBox.buttonText.ok;
        Ext.MessageBox.msgButtons['cancel'].text = Ext.MessageBox.buttonText.cancel;
        Ext.MessageBox.msgButtons['yes'].text = Ext.MessageBox.buttonText.yes;
        Ext.MessageBox.msgButtons['no'].text = Ext.MessageBox.buttonText.no;
    }
});