jsunit: Add proper lang tag to document

Signed-off-by: Julius Härtl <jus@bitgrid.net>

core/js/tests/specHelper.js

@@ -117,6 +117,7 @@ window.Snap.prototype = {
 };
 
 window.isPhantom = /phantom/i.test(navigator.userAgent);
+document.documentElement.lang = navigator.language;
 
 // global setup for all tests
 (function setupTests() {