How different was Dante's spoken dialect from the language of the Divina Commedia?

Question

It is often said that Dante invented the Italian language, but how different was the dialect Dante spoke from the language of his Divina Commedia?

Answer 1

In Italy, in XIII century, the spoken language was the vernacular language, the “official” language was Latin.
Latin, however, remained in use among a minority of educated persons, mostly priests and monks of the Catholic Church, who probably used it often as a spoken language.
So you have to consider that the spoken language was different depending on the person they were talking to.
So in everyday life, Dante spoke Latin (with literate people) or vernacular language.
Surely the language that Dante used was not too casual, you have also to consider that he was a cultivated person, so his vernacular language was surely different from that spoken by poor plebs (there were a lot of illiterate).

As @DaG said, Dante is not the “inventor” of the Italian language, but some people says that (Italian Wikipedia defines him as “The father of the Italian Language”) because his Divine Comedy, widely considered the most important poem of the Middle Ages and the greatest literary work in the Italian language, was entirely written in vernacular language, the dialect used in everyday life by the common people of his time.

Dante, in his letter “Epistola XIII” to “Cangrande della Scala” wrote (in Latin):

31. Et per hoc patet, quod comedia dicitur presens opus. Nam si ad materiam respiciamus, a principio horribilis et fetida est, quia Infernus, in fine prospera, desiderabilis et grata, quia Paradisus; ad modum loquendi, remissus est modus et humilis, quia locutio vulgaris, in qua et muliercule comunicant.  32. Sunt et alia genera narrationum poeticarum, scilicet carmen bucolicum, elegia, satira, et sententia votiva, ut etiam per Oratium patere potest in sua Poetria; sed de istis ad presens nihil dicendum est (Dante, Epistole, XIII, 31-32).”

“Therefore it is clear why this work is called Comedy. In fact, if we look at the content, initially horrible and repugnant, because it describes the Inferno, in the end it appears positive, desirable and pleasant, because it illustrates Paradise; as for the expression, a measured and humble language is used, as it uses the vulgar language in which women express themselves. But there are also other kinds of poetic narratives, such as bucolic poem, elegy, satire and votive singing, as Horace explains in his poetic Art; but, in this context, it is not appropriate to speak about”.