Why is xml:lang not allowed on typed literals?

Hi all

This list does not seem very active, but hopefully someone is still
monitoring it and will be able to answer

In
http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-Graph-Literal I
read

Plain literals have a lexical
form<http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#dfn-lexical-form>and
optionally a language
tag as defined by
[RFC-3066<http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#ref-rfc-3066>],
normalized to lowercase.

Typed literals have a lexical
form<http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#dfn-lexical-form>and
a datatype
URI being an RDF URI
reference<http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#dfn-URI-reference>
.

Between the lines I read that the language tag xml:lang is not allowed on
typed literals. Actually I just tried to do this. The rationale is to define
a datatype "One Sentence" which must contain a single sentence, starting
with a upper-case, ending with a dot etc ... and using this datatype for a
"tagLine" property - which of course has also a language.

So I tried the syntax below and proposed it to various tools

- W3C validator validates it, seems to ignore the xml:lang tag

- Protégé does the same, imports the file and ignores the xml:lang tag when
saving

- SWOOP does the other way round, ignores the rdf:datatype but keeps the
language tag.

My question is, just out of curiosity, what is the rationale behind not
allowing xml:lang on typed literals?

Thanks for any clue

Bernard

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
    xmlns:voc="http://example.org/voc#">

            <rdf:Description rdf:about="
http://products.example.org/widgets/EBSW">
                <rdfs:label xml:lang="en">Example Best Super
Widget</rdfs:label>
                <voc:tagLine xml:lang="fr" rdf:datatype="
http://example.org/datatype#OneSentence">Notre meilleur widget est le plus
beau et le moins cher du monde.</voc:tagLine>
            </rdf:Description>

</rdf:RDF>
-- 
Bernard Vatant
Senior Consultant
Vocabulary & Data Engineering
Tel:       +33 (0) 971 488 459
Mail:     bernard.vatant@mondeca.com
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