The dialog preview is always in german for me although the default language of my dialog is english. It should be possible to preview the dialog in other languages than the current DOpus language. I can't preview the english dialog to test if a combo box is large enough to show all entries of it's list.
One more thing: when I'm in language overlays mode to adjust the german dialog, changes are not applied to the preview. I have to close the language mode to see the changes and go back again.