For the sake of simplicity, I say:
1. Not only SNES, but you can add whatever letter to whatever games. PSX, PSX, PS3, PS4, Xbox, PC, Wii, NDS, 3DS or whatever you can name.
2. What is shown on the screen isn't effected by what you type with a keyboard. Characters in games relate to codes. So, to add your language characters, re-draw the existing characters first, then find the codes control these characters.
For example, you can see many "foreign characters" in the pics below. I also made some Sanskrits.
In original game, the code 1F controls the letter "A". Whenever the game hits 1F, it displays "A" on the screen.
I re-drawn the letter "A" to the yellow Sanskrit letter.
So, whenever the game hits 1F, it displays the Sanskrit which I editted from "A".