Braille key. With each Pokemon Ruby, Sapphire, Emerald, FireRed and LeafGreen version, you should obtain a Braille Key in the box. This gives you the symbols and the letters they stand for - which are used in the game. If you do not have this, it is easy to find. Wikipedia gives a Braille key under the heading "Writting Braille". Pokemon only uses the alphabetic letters and the symbols for the full stop and the comma. http://en.wikipedia.org/wiki/Braille
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There is no "braille decoder", you have to manually decode it yourself.
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It's on Six Island in that cave with the Braille.
Read the braille, then don't touch the game for about 2 minutes. This will open the door to Regice.
You need Relicanth to be first in your party, and A wailord as the last, and then when you've read the braille you need to get one of your Pokemon to learn dig.