FLD syntax question

I have a simple question about FLD syntax and semantics.

The XMLSchema says:

  <xs:element name="Const">
    <!--
  Const          ::= '"' UNICODESTRING '"^^' SYMSPACE | CONSTSHORT
    -->
    <xs:complexType mixed="true">
      <xs:sequence>
        <xs:group ref="IRIMETA" minOccurs="0" maxOccurs="1"/>
      </xs:sequence>
      <xs:attribute name="type" type="xs:anyURI" use="required"/>
      <xs:attribute ref="xml:lang"/>
    </xs:complexType>
  </xs:element>


This means that syntactically there may be an "xml:lang" with any value of
the attribute "type".
I wonder if the use of a non-empty value in "xml:lang" with any "type"
rather than
rdf:PlainLiteral (http://www.w3.org/1999/02/22-rdf-syntax-ns#text)
should be considered a semantic error?

My guess is that it should not, but I want to be sure.

If it is not an error, then do we have to completely ignore the language
tags
in non-PlainLiteral constants?
If we do, then the following objects must be considered logically identical:

<Const type="mytype" lang="en">
    abra kadabra
</Const>

and

<Const type="mytype" lang="de">
    abra kadabra
</Const>

because they only differ in the value of "lang".
This is not nice, so I guess we should distinguish them, but I would like to
be sure.

A quick answer would be appreciated, even if preliminary or just a reference

to a place in the proposals where this stuff is defined.

Cheers,
======================================
Dr. Alexandre Riazanov (Alexander Ryazanov)
Montreal, Canada
cell: +1 - 514 - 961 86 89
http://www.freewebs.com/riazanov/
http://www.linkedin.com/in/riazanov
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