Complex sentence: Corsican was long the vernacular language alongside Italian, official language in Corsica until 1859, then replaced by French due to the annexation of the island by France in 1768.
Split into simpler sentences: Corsican was long the vernacular language besides Italian, which retained official status in Corsica until 1859. Since then, it has been replaced by French due to the annexation of the island by France in 1768.