The Zu is the ancient language of the Zu Folk. The alphabet and numerals of the language are used to solve many puzzles in the game.
Decipher the alphabet
The key to deciphering the alphabet can be found in-game on a pillar which says "The quick brown fox jumps over the lazy dog" in Zu with a fox jumping over a dog in front of the podium. As this sentence uses every letter in the alphabet it is possible to decipher the entire alphabet from this. Note that the language is read up-down, then right-left.
The FEZ and the D'ni numeral systems are built very similarly, and it's possible that the system in FEZ is inspired by the one in MYST