"It is also good practice to identify the primary language of a document, either with markup (as shown below) or through HTTP headers." 
"Use both the lang and xml:lang attributes when specifying the language of an element. The value of the xml:lang attribute takes precedence." 
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> ... </html>
You should be aware of the fact that the "lang" attribute is not defined in XHTML Basic. Therefore, you should only use "xml:lang" in this case.
Copyright © 2001-2003 by Rainer Hillebrand and Thomas Wierlemann