Mint login screen: wrong keyboard layout

To solve the issue, do the following (as proposed in this comment on github):

1. Switch to a text console (e.g. using ALT+CTRL+F1) and login.
2. Open the following file using your preferred text editor (with root privileges):

   /etc/mdm/Init/Default

3. Go to the last line – it should be exit 0
4. Insert a new line before that last line and enter /usr/bin/setxkbmap de for a german localisation. Choose the localisation according your needs, for example fr for french localisation, ru for russian and so on.

Now, the last lines of /etc/mdm/Init/Default should be:

/usr/bin/setxkbmap de
exit 0

Don’t forget to adapt the file /etc/mdm/Init/Default again after upgrading to a new Mint version.

On this website you can find an overview of the abbreviations of all the layouts: