The Venetian people had their own language for the well over a thousand years the Republic of Venice existed.
The Venetian language is a Romance language but it is closer to French than to modern Italian.
Venetian, like most other italic languages, descends from vulgar Latin. The Latin common people spoke in late antiquity was very simplified compare to classical Latin. It was, however, this vernacular that became the basis of most Romance languages in the early Middle Ages.
The Change to Italian
Venetian is little spoken today, and mostly among older persons.
That is a change that has been two hundred years in the making.
Already in the late 1700s the better educated in many states on the Italian peninsula had a tendency towards a more shared, Italian like language. They travelled, traded and communicated with each other.
The Venetian travel guide depicted here, written by a courier of the Serenissima, was published in 1785. The text is immediately understandable, even if not exactly modern Italian, because he wrote for a wealthy, educated audience.
Common people, the vast majority of the population, continued to speak Venetian.
The Italian language only became dominant in Venice well after the annexation of the NE of Italy to the newly united Italian Kingdom in 1866.
The cultural pressure from the Italian language has pushed Venetian into the margins.
The Italian state has promoted the Italian language from the start, initially through mandatory military service and primary schooling, then radio and television and ultimately the internet.
Now young Venetians mostly use the Venetian language to speak to their grandparents.
If Venetian was the national language of a sovereign state for a millennium, it is now oddly a dialect of Italian, which is a much younger language.