public class MyModel extends Model {
    private Map map;

    protected void setMap(Map<Object, Object> map) {
        this.map = map;
    }

    protected List<Object> getIds(){
        return new ArrayList<Object>(map.keySet());
    }

    public String getDescription(Object id){
        return map.get(id);
    }
}

public class MyChoiceRenderer implements IChoiceRenderer {
    private Component c;
    public MyChoiceRenderer (Component c) {
        this.component = c;
    }
    public Object getDisplayValue(Object object) {
        return (MyModel)(component.getModel()).getDescription(object);
    }
    public String getIdValue(Object object, int index) {
        return object.toString();
    }
}

IModel myModel = new MyModel(){
    @Override
    public Object getObject() {
        super.setMap(service.getItems(getLanguage());
        return super.getIds();
    }
}
final DropDownChoice ddc = new DropDownChoice(id, choices);
ddc.setChoiceRenderer(new MyChoiceRenderer(ddc));