What's the 'secret language' in Fez, and can you learn it in-game?
Yes, the code can be learned through the game. However, it is not an in game mechanism, and sentences won't be automatically translated for you.
There is a special room in the game that should help you decipher the alphabet.
Deciphering this alphabet is required if you wish to finish the game to full completion, and can be done on the first playthrough.