Problems with creating DocumentType from XML schema containing xml:lang

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Hallo wmUsers!

I have a problem with importing an XML-schema, which contains an attribute reference to xml:lang.

The schema looks as follows (basically):

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema 
    xmlns="http://xyz.com/Examp" 
    xmlns:xs="http://www.w3.org/2001/XMLSchema" 
    targetNamespace="http://xyz.com/Examp"
    elementFormDefault="qualified">
    
    <xs:import namespace="http://www.w3.org/XML/1998/namespace"/> <!-- ** -->

    <xs:element name="Examp">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="Text">
                    <xs:complexType>
                        <xs:simpleContent>
                            <xs:extension base="xs:string">
                                <xs:attribute ref="xml:lang" use="required"/>
                            </xs:extension>
                        </xs:simpleContent>
                    </xs:complexType>
                </xs:element>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
</xs:schema>

So a valid XML document would look like

<?xml version="1.0" encoding="UTF-8"?>
<Examp xmlns="http://xyz.com/Examp">
    <Text xml:lang="en-us">Hello space!</Text>
</Examp>

Now, if I want to create a DocumentType from that schema, (after quite some waiting) the Developer throws the error:

Success: false

Error: 0
    identifier: /schema/element[@name="Examp"]
    
    source: -----   Source   -----
    <xs:attribute ref="xml:lang" use="required"/>
    
    errorMessage: [ISC.0082.9706] Declaration not found

What could be the problem here and how could I solve it, so that I can at the successfully validate an XML document containing the xml:lang attribute?

Thx, Christoph

PS:
I also tried modifying the schema, with the import-line looking as follows:

<xs:import namespace="http://www.w3.org/XML/1998/namespace" schemaLocation="http://www.w3.org/2001/xml.xsd"/>

or

<xs:import namespace="http://www.w3.org/XML/1998/namespace" schemaLocation="C:\xml\Schema\xml.xsd"/>

or
by even leaving the import-line away.

But none of it worked.