Complex sentence: The vast majority of Belgium's population, 99%, is literate as defined by the Belgian government, i.e. capable of reading and writing in an official language by the time a citizen has reached the age of 15.
Split into simpler sentences: The vast majority of Belgium's population, 99%, is literate. Defined, by the Belgian government, as capable of reading and writing in an official language by the time a citizen has reached the age of 15.