Speaking a language fluently does not mean one can read and write it. Literacy is a specific skill, and different cultures and societies display different levels and approaches towards it. In some, literacy is reserved for the nobility, bureaucratic or religious classes. In others it is unknown with oral traditions prevailing instead.
Naturally Literacy is specialized according to its associated language, including Native Tongue. Before once can be literate in a Language one must be able to speak it. Literacy cannot exceed the character’s related Language skill. The skill works similarly to Language in terms of how much of a document can be comprehended.
Literacy does not necessarily imply writing, as there are an infinite number of ways to record information. Some cultures might carve symbols into trees or lengths of bone, whereas others record history in complex patterns of knots in a tapestry. Neither are all forms of literacy based on phonetic alphabets; hieroglyphs and ideograms also being commonplace.