I believe the Namespaces spec is less than specific on this point, but the accepted interpretation is that it is illegal to attempt to define the "xml" prefix... it should be implicitly defined. XA and XML Spy disagreed on this, and it was raised on a W3C Schema WG discussion list, with the outcome being that it is at best bad practice to define the prefix, even if you define it to the correct namespace. The best practice is to leave it as implicitly defined.
I believe that not only is the prefix implicitly defined, it is also unnecessary to explicitly import the namespace to gain access to "xml:lang" and "xml:space".
>Hi Scott:
>
>Can you please help to answer the question of whether the following
>declaration
>
> xmlns:xml="http://www.w3.org/XML/1998/namespace"
>
>is necessary in an XML schema that wants to use the xml:lang attribute?
>
>Attached is the schema downloaded from the above URL. It clearly indicates
>that the xml: namespace prefix needs to be declared and that the namespace
>needs to be imported into the target namespace.
>
>On the other hand, both Internet Explorer and XML Authority choke on the
>
> xmlns:xml="http://www.w3.org/XML/1998/namespace"
>
>declaration. The spec at http://www.w3.org/TR/REC-xml-names/
>also suggests that the xml: prefixed namespace should have been defined
>implicitly.
>
>Thanks,
>-Arvola
>
>