In the Linux Mint 17.2 /etc/mdm/Xsession script, there's code which reads keymap name from a filein user's home directory. The contents of the file is simply the keymap name. Alternatively, there's also code to read xmodmap setup instead. So, forcing the keymap to be set to German layout should happen by running this line to generate file with contents 'de' in the user's home directory:
echo de > $HOME/.Xkbmap
Note that each user of the computer will have unique keyboard setup by having their favorite setup name in their own home directories.
This is the code in /etc/mdm/Xsession responsible for acting on the files:
156 if [ -f "$userxkbmap" ]; then
157 setxkbmap `cat "$userxkbmap"`
161 # xkb and xmodmap don't play nice together
162 if [ -z "$XKB_IN_USE" ]; then
163 if [ -f "$usermodmap" ]; then
164 xmodmap "$usermodmap"