Language Pack Plugins Cause Upgrade to Fail

2009-01-06 16:11:41,997 ERROR [main] [atlassian.confluence.event.ConfluenceEventManager] publishEvent An exception was encountered while processing the event: com.atlassian.confluence.event.events.admin.ConfluenceReadyEvent[source=com.atlassian.confluence.upgrade.impl.DefaultUpgradeManager@4544c134]
java.lang.RuntimeException: Error initialising plugin manager: Duplicate plugin found (installed version is the same or older) and could not be unloaded: 'confluence.languages.de_DE'
	at com.atlassian.confluence.plugin.ConfluencePluginManager.processConfluenceReadyEvent(ConfluencePluginManager.java:278)
	at com.atlassian.confluence.plugin.ConfluencePluginManager.onApplicationEvent(ConfluencePluginManager.java:179)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)