Jeg tolker følgende som at det er indholdet på siden og ikke selve elementerne, hvorfor jeg mener du bør angive "da" fremfor "en"
C.7. The lang and xml:lang Attributes
Use both the lang and xml:lang attributes when specifying the language of an element. The value of the xml:lang attribute takes precedence.
http://www.w3.org/TR/xhtml1/#C_12.12 Language Identification
In document processing, it is often useful to identify the natural or formal language in which the content is written. A special attribute named xml:lang may be inserted in documents to specify the language used in the contents and attribute values of any element in an XML document.
http://www.w3.org/TR/2000/REC-xml-20001006