Fix the default XML lang tag of `DefaultServerRuntimeContext` (#20)

diff --git a/server/core/src/main/java/org/apache/vysper/xmpp/server/DefaultServerRuntimeContext.java b/server/core/src/main/java/org/apache/vysper/xmpp/server/DefaultServerRuntimeContext.java
index ff1f171..05758b8 100644
--- a/server/core/src/main/java/org/apache/vysper/xmpp/server/DefaultServerRuntimeContext.java
+++ b/server/core/src/main/java/org/apache/vysper/xmpp/server/DefaultServerRuntimeContext.java
@@ -206,7 +206,7 @@
 
     @Override
     public String getDefaultXMLLang() {
-        return "en_US"; // TODO must be configurable as of RFC3920
+        return "en"; // TODO must be configurable as of RFC3920
     }
 
     public StanzaRelay getStanzaRelay() {