How to edit Drupal core rdf.module functionality
You can add the following preprocess function to template.php in your custom theme. The RDF module is preprocessing the variables used by the theme_username() function. Your custom theme can also preprocess these same variables and override the values set by the RDF module.

/*
 * Implements THEME_preprocess_HOOK().
 */  
function YOURTHEMENAME_preprocess_username(&amp;$variables) {
  $variables['attributes_array']['xml:lang'] = 'en';
}