Dante Alighieri wrote this, his most famous work, in Italian. More specifically, he wrote it in what was then a Tuscan dialect of Italian.
This is important for a couple of reasons. First of all, most serious writing in Dante's days was done in Latin. For someone to write a piece of real literature in a vernacular language was a big step.
Secondly, Italian had not yet been standardized into one language at that point. By writing this book, Dante helped make his dialect into the basis of what became a national Italian language.
It is difficult to imagine a work of literature of more importance to a national culture than Dante's The Divine Comedy. Not only did he write it in the Tuscan or Florentine Italian, this long poem helped make that dialect, or version of Italian, the standard one for Italy. However, the poem also pulled in words from other Italian dialects and other languages.
In the early 1300s when Dante wrote his masterwork, Latin was the international language widely used by educated people of Europe for their written texts. It was, therefore, unusual for Dante to write a major literary work in the vernacular, the native language of one's country, but Dante did so, along, it might be noted, with fellow medieval writers Petrarch and Boccaccio.
It's worth noting that 600 years later, Italian Primo Levi devotes a whole chapter of his book Survival in Auschwitz to regaining his sense of humanity for a short time while in the concentration camp simply by having the opportunity to recite a canto from The Divine Comedy to a fellow inmate.
According to the Italian Dante Society, the poem was constructed in Italian. No original copies exist. Dante’s Divine Comedy is considered to be one of the greatest poems in the Western world. Despite its vastness, the poem is simply structured. A Pilgrim is lost in the woods of error and is prevented from finding his way back to the path by three fierce beasts. He is rescued by the shade of the poet Virgil, who will restore him to his right path. They will take a roundabout way, through Hell, Purgatory, and Heaven. Virgil guides him through Hell and Purgatory, and then Dante’s visionary and spiritual lady, Beatrice, leads him through the nine spheres of Heaven. The poem ends with a vision of God.