xml:lang gets used if document is processed by XML tools
you can remove it if you wish
and please do provide the validator link where this is showing an actual error
I would liek to test more before 2.0.1
"The xml:lang attribute is not actually useful for handling the file as HTML, but takes over from the lang attribute any time you process or serve the document as XML. The lang attribute is allowed by the syntax of XHTML, and may also be recognized by browsers. When using other XML parsers, however (such as the lang() function in XSLT) you can't rely on the lang attribute being recognized."
Declaración de idioma en HTML