thanks for the reply. Currently I can provide these information:
It seems, that the encoding or font in Xfce is related to the used gkt2-engine. I used the axiom theme, which provides in all gtk based apps a clear ui (I think the standard font is sans).
Strangly OpenOffice/LibreOffice and Skype, that I installed using aptitude, showed Greek letters instead of the latin ones. In Skype the whole ui was Greek. In OpenOffice were only the menu-items in the wrong font - everything which was typed, was in a latin font.
Removing skype and using the static pakage solved the issue for this application.
For OpenOffice, I didn't found the solution, but by changing the theme solved the issue.
Maybe some knows, what problem lies behind this behavior and explain it. For everyone else with this problem, I hope the "workaround" would fix the issue.