You cannot find a Braille decoder sheet in any of the Pokemon games. Braille is very simply and easy to translate if you print off a Braille alphabet sheet for reference.
Look up a normal braille sheet on the internet
There is no "braille decoder", you have to manually decode it yourself.
Sadly, the game itself provides no translation sheet. However, if you take a look at one of the game's many game guides (either in print or online) there should be a decoder sheet on any source worth its salt.
Using a braille decoder.
Go to wikipedia, they have a braille dictionary.
braille is a way for blind people to read. you need to get the decoder to read it.
The Braille Decoder sheet for unlocking the Regis in FireRed and LeafGreen was sold in the official guide released by Prima for the games. The Braille used in the games as a visual representation of the raised bumps and spaces of the braille language.
A braille reader does not exist in Pokemon Ruby or any Pokemon game with braille used as a puzzle. Translating the braille for yourself is very simple if you print off a braille alphabet sheet.
You cannot learn the Braille alphabet in the Pokemon games. You will have to translate the Braille yourself by using a Braille alphabet sheet. This is very simple and easy to do with a pen and paper.
It is not possible to get a Braille Dictionary in Pokemon Emerald. The Braille in-game is easy to decode as it translates into English. Decoding the Braille is as simple as using a pen and paper along with a printed Braille sheet for reference.
Those symbols are braille letters. To decode them, just search "braille decoder" in google. They give you instructions on what to do to get into the next room. In the next room is one of the regis. (Regice, Regirock, Registeel)