Short answers, pulled from the story.
Latin and Greek were the two dominant languages of the Roman Empire. Latin was the language of imperial administration, legislation, and the military throughout the classical period, while Greek was the common language of the eastern Mediterranean and a legacy of Alexander the Great's conquests. Regional languages including Aramaic, Coptic, Punic, Celtic dialects, and Gothic were also spoken across the Empire.
There was never an official language of the Roman Empire. Latin was the language of government, law, and the military, but it was not formally imposed on conquered peoples. Saint Augustine observed that Romans preferred for Latin to be adopted per pacem societatis, through a social pact rather than decree.
Latin spread from the 2nd century BC onward, particularly in the western provinces. Classical languages expert Bruno Rochette places the major period of spread from the second century BC, with especially strong expansion in the west. People adopted Latin voluntarily because fluency carried high cultural, political, legal, social, and economic value, particularly for imperial service and advancement.
Greek played a central role in spreading Christianity because it was already the international language of the eastern Mediterranean. The New Testament was written in Greek, the Epistles of Paul circulated in Greek, and the ecumenical councils of the Christian Roman Empire were conducted in Greek rather than Latin. The use of Greek at the Council of Nicaea meant Constantine, who had Latin as his court language, communicated with Greek-speaking bishops through an interpreter.
After the decentralization of political power in late antiquity, Latin developed locally in the western provinces into the Romance languages, including Spanish, Portuguese, French, Italian, Romanian, Catalan, Sardinian, Aromanian, and others. By the early 21st century, the first or second language of more than a billion people derived from Latin. Greek continued as the language of the Byzantine Empire and eventually developed into modern Greek.
Gaulish survived well into the late 6th century, centuries after Roman conquest. Historical linguistics demonstrates that Gaulish coexisted with Latin throughout the entire era of Roman rule. Evidence in the writings of Gregory of Tours, who lived from 538 to 594, indicates that Gaulish may still have been spoken in France during his lifetime.
The Regina epitaph is a 2nd-century gravestone discovered in 1878 near the Roman fort at South Shields in northeast England. It commemorates a woman named Regina and is written in both Latin and Palmyrene Aramaic by her husband Barates, a standardbearer from Palmyra, Syria. The inscription shows at least four linguistic layers: Latin, Palmyrene Aramaic, grammar influenced by Greek honorific conventions, and the name Regina itself, which can be either Latin or Celtic.