The examples of personal correspondence we have, such as between Cicero and Atticus and between Augustus and his family, are all in Latin. Greek words and phrases would be dropped liberally throughout but the primary language was very much Latin. Furthermore, Cicero feels it neccesary to encourage the learning of Greek and to justify his philosophical works by saying many people would otherwise miss out on Greek philosophy due to lack of proficiency with the language.

In fact I generally think think that the prevalence of Greek in the upper classes has often been exaggerated by people who take Cicero as a representative elite, rather than an exceptionally well educated one.