How to add content-language
Hope this will help you,

In Drupal 7, you can achieve it using template_page_alter

    function THEME_NAME_page_alter($page) {
        global $language;
        $lang_name = $language->language;
        $meta_language = array(
          '#type' => 'html_tag',
          '#tag' => 'meta',
          '#attributes' => array(
            'http-equiv' => 'content-language',
            'content' => $lang_name
          )
        );
        $meta_content_language = array(
          '#type' => 'html_tag',
          '#tag' => 'meta',
          '#attributes' => array(
            'http-equiv' => 'language',
            'content' => $lang_name
          )
        );
        drupal_add_html_head($meta_language, 'content-language');
        drupal_add_html_head($meta_content_language, 'language');
    }



Folder listing of active theme folder


template_page_alter inside template.php file

  


Source