Multiple keyboard and input languages on raspbian

I haven't tested this, but I assume that you can change the keyboard layout in the same way as on a normal Debian install. Raspbian uses LXDE as its desktop environment, so you can follow their instructions:

setxkbmap -option grp:switch,grp:alt_shift_toggle,grp_led:scroll us,th

See http://wiki.lxde.org/en/Change_keyboard_layouts (and also http://wiki.debian.org/Keyboard) for more information.