Darth Vader’s chestpiece is an ancient Hebrew writing which translates to “His deeds will not be forgiven, until he merits.”This sounded too good to be true, and a little checking indicates that it is only partly true. In the movies and elsewhere there are various versions of Darth Vader's chestpiece, most of which bear strings of Hebrew letters, sometimes upside-down and/or in odd fonts. But none of these make any sense. It looks as though random Hebrew letters were used to make the writing look exotic. It occurs to me also that the chestpiece may be intended to evoke the High Priest's breastplate in Exodus.
The most comprehensive discussion I could find of the writing on the chestpiece is here. And I see that, not surprisingly, James McGrath already discussed the topic several years ago, and his commenter Jason Staples noted the parallel with the High Priest's breastplate.