Lubuntu is the most lightweight distribution, I ever seen before.
It is so light, that some basic functionality is missing 😉
One of the problem, I met was possibility to switch keyboard layout for various languages. There is no such highly usable keyboard language switcher, like it is in Gnome or KDE.
Google helped in this case pretty fast.
This Ubuntu forum post gave me a very simple answer.
Run the following command line
echo '@setxkbmap -option grp:ctrl_shift_toggle "us,ru"' | sudo tee -a /etc/xdg/lxsession/Lubuntu/autostart
Note: in case you are used to Alt+Shift button combination, use alt_shift_toggle setting.
Then restart X Server and add to panel Language indication.