Squirt lighter fluid onto the glass where the painted lettering is. Lighter fluid is not as harsh as some other chemicals and can be used on many surfaces without causing damage.
Allow the lighter fluid to sit on the surface for 15 minutes. This will soften the paint so you can wipe most, if not all of it, away.
Use a dry rag to wipe away as much of the paint as you can. This will require some tough scrubbing. If all the paint isn't removed after doing this, move on to Step 4.
Pour acetone on a rag and wipe the areas that still have painted lettering visible. You may need to do this twice. If the rag gets saturated with too much paint, get a clean one. This will remove more paint instead of just moving it around.
Use a small paint scraper if painted lettering still shows on the glass surface. Be very careful not to scratch or chip the glass.