The labels on the properties dialog are read directly from the appschema.json file of the individual app.
None of the json configuration files currently have a mechanism for translation. This will be considered for a future release.
Maybe the solution with the templates for each language as for some other Portal item this would done could solve the Problem. - see http://earlyadopter.esri.com/project/feedback/view.html?cap=0A4390EB2493466F8AE12E5BF9B488D3&uf=6D71...
The link you're posting to the early adopter site is not working.
My guess is that you are suggesting that we create different template item in ArcGIS Online for each language - is that correct?