Did you have a look at the Debug Output Windows as suggested by Chris?
If you see a message like this
XmlDataProvider cannot load asynchronous document from Source because of load or parse error in XML stream. Uri:'Languages.xml' IOException:'System.IO.IOException: Cannot locate resource 'languages.xml'.
then the XML file cannot be found.
In this case you can try to assign a path like this:
<XmlDataProvider x:Key="XmlData" Source="c:\\Languages.xml" XPath="Languages"/>
Do not forget to copy the XML file to the specified path.