externalizing property files does not work?

Check your portlet.xml for NameScopeServletContext

 
<init -param>
<name>NamescopeServletContext</name>
<value>true</value>
</init>

if NameScopeServlet context is true,
you will have to replace the bundle name with <portlet-name>+bundle name
for example if my portlets name is "test portlet" then the bundle name becomes

 bundle="test portlettestkey"

where testkey is the actual bundle name specified in struts config.(note that space if present should also be included)

The reason for this behavior is that NameScopeServletContext is used to specify if the web application has multiple struts portlets/apps. so if NamescopeServletContext is set as true the name of the application should also be specified in addition to the bundle name so that it knows the bundle is for which application.