Install this package.
#apt-get install ttf-bengali-fonts ttf-indic-fonts
you may need to restart your Xserver to view bangla/bengali.
System–>Preferences–>Keyboard
Click Add
Layout Option—>Group Shift/Lock behaviour (or use whatever you like)
Now I can switch to Bangla/English by pressing two ALt key together.
(Use any option here that you like)