There's an example on the Arduino Project Hub that I found via a simple Google search. It shows how to read from a text file stored on an SD card 1 character at a time and display it on a LCD.
It should be fairly simple to modify the example to process the characters.
About 1000 years ago (or so it seams now!) when I used to muck about with large format plotters, the language used to speak to them was called HPGL (Hewlett-Packard Graphics Language). It's a really simple text based language. Have a read about it on Wikipedia as it would save you having to invent your own language (unless you want to. There's also some HPGL viewers out there so you can visualise the results on screen as well.