Change default filename on export

[ChrisBlankDe]

Currently the default filename does only contain the country part of an language code:

MyApp.fr.xliff

Please change it to contain the full language code:

MyApp.fr-ca.xliff

Thats the way we have to provide these data to Microsoft and its necessary to difference multi country languages like spanish (sp+mx), french (be+ca+ch+fr), english (au+ca+gb+nz+us+za), german (de+ch+at), ....