I'm not sure about *every* sign in the game, but the vast majority are readable. It's 90% English letters with a few reversed letters and odd shapes (eg. the "HQ" item symbol is actually an Eorzean 'S'). You can decipher it by looking at the maps and comparing the symbols to what you know the label should say.
My only other thought is that a Lalafell player might be able to get a closer look at the gravestone in first-person view.
