http://issues.apache.org/jira/browse/JS2-174: Support Tomcat TCP Cluster Session attributes needs to be Serializable. In this case, the PortalReservedParameters.PREFERED_LANGUAGE_ATTRIBUTE (WeakHashMap) doesn't seem to be used anymore, so I simply removed it.
portals/jetspeed-2/trunk/components/portal/src/java/org/apache/jetspeed/request/JetspeedRequestContext.java