A Keyboard Driver is a piece of software that allows you show on screen, say a letter "x" when you press a key "x" on your keyboard. It seems obvious that it should be so but you are pressing an electrical switch and a letter from a font is digitized and rendered on screen. What is more, when you press simultaneously a key "x" and key "Shift" on your keyboard, a letter capital "X" appears. And if you press a key "x" and key "Control", usually something gets deleted. It is clear that there is software that maps your key presses to some actions on your computer.
If you set your computer to be used in English you automatically chose an English Keyboard Driver. If you wish to type in a foreign language, say Russian, you would need a software that maps letters in that language to some keys or combination of keys that would be natural in that language - that would be a Russian Keyboard Driver.
We developed a set of Keyboard Drivers for a number of languages but since Microsoft later developed them as well and included them with Windows, there is no much point in duplicating them so we discontinued them. But if you buy a font for a foreign language from us or anybody else, you would need to install a Keyboard Driver for that language in order to use the font successfully - a font by itself does not change the way your keyboard works.