Get element description from XML.

def get_element_desc(element, ar_tree, ns):
    # type: (_Element, _DocRoot, str) -> str
    """Get element description from XML."""
    desc = get_child(element, "DESC", ar_tree, ns)
    txt = get_child(desc, 'L-2[@L="DE"]', ar_tree, ns)
    if txt is None:
        txt = get_child(desc, 'L-2[@L="EN"]', ar_tree, ns)
    if txt is None:
        txt = get_child(desc, 'L-2', ar_tree, ns)
    if txt is not None:
        return txt.text
    else:
        return ""