These days I was confused about why I could not apply to a standard user the keyboard-layout configuration I had been able to add and apply as root to the Login-screen (To do so, I applied the debian.org guide).The problem I thought I had was authorization, but in fact the setup for /etc/default/keyboard was fine (after I reconfigured it as I mentioned above). This is how it looks:# KEYBOARD CONFIGURATION FILE# Consult the keyboard(5) manual page.XKBMODEL=pc104XKBLAYOUT=latamXKBVARIANT=XKBOPTIONS=lv3:ralt_switch,terminate:ctrl_alt_bkspBACKSPACE=guessLooking through forums and other StackExchange questions I found that quite a few people have problems with this (add extra keyboard-layouts), and that there are different guides basically because there are different Desktop Environments (DE).
Debian Stretch gnome DE: Add keyboard layout
debian;gnome3;keyboard layout;settings