The Translation Key can be picked up from the top of the temple entrance in the Ancient Ruins of Shark Tooth Island. It is used to help you figure out the order of symbols to press when opening a door in the Temple Ruins.
The letters B (torn partially), Q, and S are shown on the Translation Key, but they are unnecessary in solving the puzzle.
You do not technically need to have this item if you know exactly which glyphs to press at the door entrance.
To unlock the door, press the glyphs for the letters O, P, E, then N. (the code is "OPEN")