Is anyone writing virtual keyboards for foreign alphabets? Do they already exist?

v on Ubuntu 1604.

I would especially like Greek and Hebrew.

Which desktop environment do you use? On KDE under System settings → Input devices → Keyboard → Distribution you can add as many keyboard maps as needed and even set a global keyboard shortcut to switch among them. Other desktops like gnome, etc should be similar.

OOoFBTools includes virtual keyboards for some sets of symbols, Greek and Hebrew as well.