[Antoninus]

Justinian was the last Roman emperor to speak Latin.

That is correct. It's fascinating to realize that even as late as the 6th century AD, works such as Justinian's Law Code and Novels were written entirely in Latin -- at least at first before being translated into Greek for the benefit of the majority of the Eastern Empire who were Greek-speaking.

[Romulus]

Latin was the only logical choice, as the entire jurisprudential record was in that language.