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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
or
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1 plus MathML 2.0 plus SVG 1.1//EN" "http://www.w3.org/2002/04/xhtml-math-svg/xhtml-math-svg.dtd">
and so on, not the human language of the content. The snippet of code which you provided is not part of the DOCTYPE.
In XHTML, you can specify content language in four places. Your example uses one of them, correctly: a lang attribute on the <html> element (and since you are presumably serving the document as MIME type text/html instead of application/xhtml+xml, the additional namespace declaration and xml:lang attribute are also correct). For the other ways or a more thorough explanation, see [w3.org...] .