A few minutes after posting the question, I found my answer in this thread. The answer basically sais the GridViewColumn wont be added to the visual tree, so bindings which uses this visual tree (e.g. FindAncestor) cant work.
So I subsribed to the Loaded-events of the elements which I bind to (e.g. GridViewColumnHeader) and did the binding in Code-behind:
BindingOperations.SetBinding(sender as GridViewColumnHeader, GridViewColumnHeader.ContentProperty, new Binding("Sprache") { Source = this, UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged, Mode = BindingMode.OneWay });