One of the articles mentioned inscriptions in Coptic, Latin, and Greek. The elites would have spoken both Greek and Latin, I think? Greek was commonly known and used. Caesar, a Very Latin Roman, as he was being murdered in Rome, is conjectured to have spoken Greek to Brutus, another Very Latin Roman.
The notion that people are mostly monolingual, and that language is very closely tied to a cultural identity, is a modern projection (and far from universally true nowadays as well).
Broadly Latin was the main language in the western Roman Empire but educated people would also have spoken Greek, and in the eastern Mediterranean it was reversed (almost everything was done in Greek but Latin was used for official purposes and by educated people).
Coptic is the interesting one though because it's actually the latest version of ancient Egyptian
The notion that people are mostly monolingual, and that language is very closely tied to a cultural identity, is a modern projection (and far from universally true nowadays as well).