Murals, Relics, and Documents can be found throughout the game, all of which give you XP and increase Lara's knowledge of ancient languages. The more you find, the more likely Lara can continue to read them.
Monoliths, however, will require a certain level, such as Fluent or Proficient, in the three languages available (Mam, Quechua, and Yucatec). The more you are able to increase Lara's knowledge, the more monoliths she can read.
If the requirements are met, then the monoliths will reveal clues to the location of hidden treasure.