If you don't have Gedit, then install it:

sudo apt-get install gedit

Open /etc/X11/xorg.conf :

gksudo gedit /etc/X11/xorg.conf

and change the following line from:

    Option        "XkbLayout"    "us"

to

    Option        "XkbLayout"    "us,bg"
    Option        "XkbOptions"    "grp:switch,grp:alt_shift_toggle"

This will add Bulgarian keyboard layout and you will be able to change your layouts pressing Shift and Alt buttons simultaneously.