Issue #673020 by mgifford, attiks: Added the Content-Language HTTP header to the generated page.

includes/bootstrap.inc

@@ -2479,6 +2479,7 @@ function get_t() {
  * Initialize all the defined language types.
  */
 function drupal_language_initialize() {
+  global $language;
   $types = language_types();
 
   // Ensure the language is correctly returned, even without multilanguage
@@ -2498,6 +2499,9 @@ function drupal_language_initialize() {
     // environments.
     bootstrap_invoke_all('language_init');
   }
+
+  // Send appropriate HTTP-Header for browsers and search engines.
+  header('Content-Language: ' . $language->language);
 }
 
 /**