Comment 7 for bug 997140

Similar issues with Mate17 set up with UK English keyboard. The login screen appears to use US (@ and " are transposed) but after login input revents to UK English. Modifying XKBLAYOUT and/or XKBVARIANT in /etc/default/keyboard per Christian's post did not make any difference. Default values were

XKBMODEL="pc105"
XKBLAYOUT="gb"
XKBVARIANT=""
XKBOPTIONS=""

what does work (for me) is editing file /etc/mdm/Init/Default

adding the line

/usr/bin/setxkbmap gb

immediately before the last line in the file, which is

exit 0

see http://unix.stackexchange.com/questions/136382/mint-login-screen-wrong-keyboard-layout