preseeding stops working on serial console in Karnic

[Josef Wolf]

Hello,

I have created a customized Karmic alternate CD where I have preseeded all
the questions for an unattended install with the following settings in
isolinux.cfg

  append  debian-installer/locale=de_DE.UTF-8 console-setup/layoutcode=de \
     console-setup/variantcode=nodeadkeys file=/cdrom/preseed/server.seed \
     initrd=/install/initrd.gz quiet DEBCONF_DEBUG=5 --

This works great so far.

But when I add console=ttyS0,9600n8 to do the install with serial console,
d-i starts asking questions again:

   [!!] Choose language

   Please choose the language used for the installation process. This
   language will be the default language for the final system.

   Choose a language:
          C
          English

     <Go Back> 

Please note that I am offered only "C" and "English" here, while without
the console=ttyS0,9600n8 setting, I can preseed german locale without any
problems, and I am not asked this question at all.

Any ideas what happens here?