This is the essense of the problem:

dpkg: error processing /home/buildbot/buildbot/debs/binary/mariadb-server-5.1_5.1.49-mariadb82_i386.deb (--unpack):
trying to overwrite '/usr/share/mysql/german/errmsg.sys', which is also in package mysql-server-core-5.1 5.1.48-1

Debian 6 splits part of the mysql-server-5.1 package into a separate
mysql-server-core-5.1 package, which causes the conflict as the MariaDB
packages do not handle this correctly with conflicts/replaces.

Note that this same problem was already solved for Ubuntu. So the fix is
merely to apply the same fix to the Debian packages (the fix does a similar
split into mariadb-server-core-5.1). As part of the fix, this part of the
change should be consolidated as far as possible so that a single shared file
is used for Ubuntu+Debian, rather than separate
bakery/debian-5.[12]/dist/

.