Arabic
Arabic is a classic case of diglossia. The standard written language, Modern Standard Arabic, is based on the Classical Arabic of the Qur'an, while the modern vernacular dialects (or languages) —which form a dialect continuum reaching from the Maghreb in North Western Africa through Egypt, Sudan, and the Fertile Crescent to the Arabian Peninsula and Iraq—have diverged widely from that. The difference between the written standard and the vernaculars is also apparent in the written language and so children have to be taught Modern Standard Arabic in school in order to be able to understand it.
Read more about this topic: Dialect Continuum