Oops, I am no longer sure the bug depends on the "Language=" line. After changing the line to "Language=" only, I had my umlauts back in one KDE session. But on another virtual display, I had the same problem again so I wonder what I did wrong or if my workaround maybe simply is not reliable. However, I reverted to kdm_4%3a3.3.0-1_i386.deb and the problem is gone completely so the bug is related to a changed behaviour of the latest kdm. Regards, Enver