LibWeb: Support for Content-Language HTTP header

Author: https://github.com/pbrw
Commit: c9edb6ffc4
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3255
Reviewed-by: https://github.com/shannonbooth ✅
Reviewed-by: https://github.com/tcl3 ✅

Libraries/LibWeb/DOM/Element.cpp

@@ -3670,6 +3670,10 @@ Optional<String> Element::lang() const
         }
 
         //      - If there is no pragma-set default language set, then language information from a higher-level protocol (such as HTTP),
+        if (document().http_content_language().has_value()) {
+            return document().http_content_language();
+        }
+
         //        if any, must be used as the final fallback language instead.
         //      - In the absence of any such language information, and in cases where the higher-level protocol reports multiple languages,
         //        the language of the node is unknown, and the corresponding language tag is the empty string.