EDIT

import eu.etaxonomy.cdm.model.description.CommonTaxonName;

import eu.etaxonomy.cdm.model.description.DescriptionElementBase;

import eu.etaxonomy.cdm.model.description.TaxonDescription;

        //common name

        Element deutscheNamen = new Element(BfnXmlConstants.EL_DEUTSCHENAMEN);

        taxonym.addContent(deutscheNamen);

        int sequenz = 1;

        Set<TaxonDescription> descriptions = taxon.getDescriptions();

        for (TaxonDescription taxonDescription : descriptions) {

            Set<DescriptionElementBase> elements = taxonDescription.getElements();

            for (DescriptionElementBase descriptionElementBase : elements) {

                if(descriptionElementBase.isInstanceOf(CommonTaxonName.class)){

                    Element dName = new Element(BfnXmlConstants.EL_DNAME);

                    Element trivialName = new Element(BfnXmlConstants.EL_TRIVIALNAME);

                    deutscheNamen.addContent(dName);

                    dName.addContent(trivialName);

                    dName.setAttribute(new Attribute(BfnXmlConstants.ATT_SEQUENZ, String.valueOf(sequenz)));

                    trivialName.addContent(((CommonTaxonName)descriptionElementBase).getName());

                }

            }

            sequenz++;

        }