Hi, Kostas!
Now I got a Greek keyboard layout on my SliTaz 4.0. Unfortunately I can not test this method on "virgin" distribution, but you can try. You need these packages: glibc-locale and xorg-setxkbmap.
Open your terminal and type (as regular user, not as root):
or
setxkbmap "us,el" -option "grp:ctrl_shift_toggle, grp_led:scroll"
Now you should immediately have two keyboard layouts: U.S. English and Greek. The second version adds a keyboard switch on the [Ctrl] + [Shift] and Scroll LED on the keyboard lights up when changing the layout (the second in the list).
"us,el" - default layout for new windows is U.S. English, press [Ctrl] + [Shift] to swith to Greek, and press again to switch back.
"el,us" - default layout is Greek, switching the same.
Have a nice day!