RFE: Add level 1 validation of @lang and @xml:lang

SVG and HTML are very similar w.r.g. how the language property of an 
element is defined and works:

1) each have two language property attributes, a “native“
    'lang' attribute plus the generic '6' attribute.
2) each permits <foo xml:lang="en" lang="en"/>
3) each forbids <foo xml:lang="fr" lang="en"/>
    (the atributes must have the same content)
4) when consumed as XML, each spec allows
    <foo xml:lang="fr" /> and
    <foo     lang="en" /> as well as
    <foo xml:lang="en" lang="en"/>
5) Both gives priority to xml:lang, when consumed as XML.
6) Unlike SVG, HTML may also be consumed as text (text/html),
    and then only the last two options are permitted:
    <foo     lang="en" /> as well as
    <foo xml:lang="en" lang="en"/>

SVG2: https://www.w3.org/TR/SVG2/struct.html#LangSpaceAttrs
HTML5: 
https://html.spec.whatwg.org/multipage/dom.html#the-lang-and-xml:lang-attributes

As showwn by validator.nu and validator.w3.org, the above can be 
validated: Both of them offers what I would call 'level 1' validation, 
by what I mean that they check whether the above rules are followed. And 
in addition, when 'level 1' is passed, they offer 'level 2' validation, 
by which I mean that they checks whether the content of the attributes 
matches the usage rules for the registered language tags.

XMLmind currently offers no validation of lang and xml:lang - neither 
level 1 nor level 2. I hereby suggest that 'level 1' validation is 
added. (Level 2 validation would of course also be welcome, but is far 
less fundamental).

What should be the consequences? I my view, it should be that,

a) when someone works with a HTML document (as opposed to a XHTML 
document), adding xml:lang without also adding lang, should trigger a 
warning, or error. (Warning can be justified since it could be that a 
HTML document will be consumed as XML - after all, XMLmind is an XML 
editork.)

b) if someone specifies both xml:lang and lang but with different 
values, an error should be displayed.

Leif Halvard Silli

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