LightDM assumes there's only ONE system default layout

Status changed to 'Confirmed' because the bug affects multiple users.

Here's a list of the affected countries. All these people can't type national characters unless they know how to go to gnome control center and add their keyboard layout and their preferred switching method.

This is a serious regression, it used to work fine in gdm and in gdm2.

$ grep XKBLAYOUT=us, keyboard-configuration.config | sort -u
 XKBLAYOUT=us,am # Armenia
 XKBLAYOUT=us,ara # Arabic
 XKBLAYOUT=us,bd # Bangladesh
 XKBLAYOUT=us,bg # Bulgaria
 XKBLAYOUT=us,bt # Bhutan
 XKBLAYOUT=us,by # Belarus
 XKBLAYOUT=us,ge # Georgia
 XKBLAYOUT=us,gr # Greece
 XKBLAYOUT=us,il # Israel
 XKBLAYOUT=us,in # India
 XKBLAYOUT=us,ir # Iran
 XKBLAYOUT=us,kg # Kyrgyzstan
 XKBLAYOUT=us,kh # Cambodia
 XKBLAYOUT=us,kz # Kazakhstan
 XKBLAYOUT=us,la # Laos
 XKBLAYOUT=us,lk # Sri Lanka
 XKBLAYOUT=us,ma # Morocco
 XKBLAYOUT=us,mk # Macedonia
 XKBLAYOUT=us,mn # Mongolia
 XKBLAYOUT=us,np # Nepal
 XKBLAYOUT=us,ru # Russia
 XKBLAYOUT=us,si # Sri Lanka
 XKBLAYOUT=us,th # Thailand
 XKBLAYOUT=us,tj # Tajikistan
 XKBLAYOUT=us,ua # Ukraine

For Greek users, we created live CDs for Ubuntu 12.04 to work around the problem:
http://ts.sch.gr/wiki/Linux/LiveCD

This might also affect Dvorak users.

I can confirm that this is a pretty big problem if you have two users who use different keyboard layouts. One of them simply cannot really enter his password and has to learn how to type it on a different layout.

Ubuntu 16.10 it is even worse, Even for a single user I can not unlock or suspend/shutdown resumed PC if I was on different layout than my password.

2017-06: Quite a long time for such a bug to survive.
As Anatoly said, it is worse on 16.10 .
If you lock the session with wrong layout, you are doomed.
Is there any hint where we should search for a solution?
Do you have any place in code to try to debug?

One more hint on this:
The button for choosing layout does not actually work, at least not when one locks the screen from the "wrong" layout.

These seem related (and none fixed) :
https://bugs.launchpad.net/lightdm/+bug/1051288
https://bugs.launchpad.net/lightdm/+bug/1343582
https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/919200
https://bugs.launchpad.net/ubuntu/+source/lightdm-gtk-greeter/+bug/990214

I have Hungarian and English keyboard layouts set up.
However at the login screen it's English and Afghan. Have no idea why. It changed at some point during 18.04.

$ grep XKBLAYOUT /etc/default/keyboard
XKBLAYOUT="us,af"

It turns out I don't have this issue anymore after a clean install of 19.10.

Are you still using lightdm there? Ubuntu 19.10 uses GDM; only some other flavors like MATE, Xubuntu and Lubuntu use lightdm.