Common Site Review Errors - Language Declaration
This error shows up second only to alt tag lapses. Although it may be low on the priority check list, it can be fixed with some simple added coding.
According to HTML Dog: You should identify the primary language of a document either through an HTTP header or with the xml:lang attribute inside the opening html tag. Although this is not necessary to produce a valid XHMTL document, it is an accessibility consideration. The value is an abbreviation, such as 'en' (English), 'fr' (French), 'de' (German) or 'mg' (Malagasy).
The declaration for a document with primarily English content for example would look like this: <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
webref.info has a page of acceptable declarations for your use depending on the DOCTYPE version you use on your pages.