Language Pack variants are not working

When defining a Resource Plugin providing two properties files:

com/atlassian/confluence/core/ConfluenceActionSupport_de_DE.properties
com/atlassian/confluence/core/ConfluenceActionSupport_de_DE_ABCDEF.properties

In case of java.lang.ResourceBundle semantic every lookup is made through de_DE_ABCDEF, in case no value for a key was found a second lookup is made using the de_DE bundle.

Unfortunately Confluence is not following this common practice. Only ConfluenceActionSupport_de_DE.properties strings are being used.