The idea that ancient Rome was bilingual or that Greek was just as important as Latin is waaaaay overstated, for several reasons:
- Since only a very small proportion were actually educated in Greek, Rome as a whole can't be considered Greek-speaking. By way of analogy, educated Englishmen in the 18th century were expected to be fluent in French, but nobody claims that 18th-century England was actually a French-speaking country, or that French was just as important as English.
- Even if Roman elites were fluent in Greek, Latin was still clearly their first language -- they communicated with each other in Latin, conducted official business in Latin, wrote books in Latin, and so on. Although they could communicate in Greek if they wanted to, the fact that, left to their own devices, they preferred to use Latin, indicates that they were Latin-speakers, not Greek-speakers.
- By the later Empire, even educated Westerners weren't fluent in Latin. So even if you want to argue that Augustan Rome was bilingual, fourth-century Rome wasn't, making the claim that switching to Greek doesn't really represent an important change more difficult to sustain.
- Even if the only difference between the Western and Eastern halves of the Empire was the language they used, language is such an important part of culture that I think a good case could be made that Greek culture was still separate to Latin.
- Since only a very small proportion were actually educated in Greek, Rome as a whole can't be considered Greek-speaking. By way of analogy, educated Englishmen in the 18th century were expected to be fluent in French, but nobody claims that 18th-century England was actually a French-speaking country, or that French was just as important as English.
- Even if Roman elites were fluent in Greek, Latin was still clearly their first language -- they communicated with each other in Latin, conducted official business in Latin, wrote books in Latin, and so on. Although they could communicate in Greek if they wanted to, the fact that, left to their own devices, they preferred to use Latin, indicates that they were Latin-speakers, not Greek-speakers.
- By the later Empire, even educated Westerners weren't fluent in Latin. So even if you want to argue that Augustan Rome was bilingual, fourth-century Rome wasn't, making the claim that switching to Greek doesn't really represent an important change more difficult to sustain.
- Even if the only difference between the Western and Eastern halves of the Empire was the language they used, language is such an important part of culture that I think a good case could be made that Greek culture was still separate to Latin.