[XXE] combo-box to list attribute values of an element

[Steve Deal]

I am still learning CSS for XXE and I am trying to use a combo-box in
CSS to list an enumeration of attributes of an element.   How do I
reference the values of the element attribute for the combo-box?

The XML would look like this:    <languages lang="fr"/>

Given the schema snippet:
        <xs:element name="languages" type="languages_ctype"/>

        <xs:complexType name="languages_ctype">
                <xs:sequence>
                        <xs:element name="language" type="my_lang_ctype" 
maxOccurs="unbounded"/>
                </xs:sequence>
        </xs:complexType>

        <xs:complexType name="my_lang_ctype">
                <xs:attribute name="lang" type="my_lang_enumeration" 
use="required">
                </xs:attribute>
        </xs:complexType>

        <xs:simpleType name="my_lang_enumeration">
                <xs:restriction base="xs:string">
                        <xs:enumeration value="ar"/>
                        <xs:enumeration value="en"/>
                        <xs:enumeration value="de"/>
                        <xs:enumeration value="fr"/>
                        <xs:enumeration value="pt"/>
                </xs:restriction>
        </xs:simpleType>

My CSS looks like this:
languages {
    display: block;
    content: element-name() "=" combo-box(attr(lang));
    color: #0000FF;
    margin: 1.33ex 0;
} 

Should I be using:  languages[lang]  or something like that?

Thanks!