I finally managed to get the installer working (Thanks to the OTN discussion forums). The trick described there indeed works: simply temporarily rename /usr/X11R6/lib/X11/locale and symlinking /emul/ia32-linux/usr/X11R6/lib/X11/locale to /usr/X11R6/lib/X11/locale.
Then, the installer can be successfully started.
I then run over another Oracle-Debian incompatibility (but I already observed this on i386 as well, but stoped investigating back then because I had an older installation which was still running): The installer stops when linking ntcontab. There seem to be some sed incompatibilities, since the installer logfile contains output like
INFO: /bin/sed: -e Expression #1, Character 7: unterminated `s' command
The installer then hangs around for ever without utilizing CPU time. I saw that there are some make and grep processes hanging around, and it seems that the grep process waits for input but does not get it, causing it to wait forever. So I simply killed the make and the grep processes (I had to do this several times for the grep process) until the installation continued.
The net configuration assistent failed to link and to run, so I had to manually configure sqlnet.ora and tnsnames.ora. Result: the client applications like sqlplus are running well!