api/xml/xml.xsd

<?xml version='1.0'?>
<xsd:schema targetNamespace="http://www.w3.org/XML/1998/namespace"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xml:lang="en">

  <xsd:attribute name="lang" type="xsd:language">
  </xsd:attribute>

  <xsd:attribute name="space" default="preserve">
    <xsd:simpleType>
      <xsd:restriction base="xsd:NCName">
        <xsd:enumeration value="default"/>
        <xsd:enumeration value="preserve"/>
      </xsd:restriction>
    </xsd:simpleType>
  </xsd:attribute>

  <xsd:attributeGroup name="specialAttrs">
    <xsd:attribute ref="xml:lang"/>
    <xsd:attribute ref="xml:space"/>
  </xsd:attributeGroup>

</xsd:schema>