Language choice: Plutarch's Life of Brutus on Romans' use of Greek

Plutarch comments on Brutus' letters in Latin and those in Greek and notes the following instances of the use of Greek by Brutus and his circle.

2.4-8:
Empylus also, who is often mentioned by Brutus himself in his letters, and ls by his friends, as a housemate of his, was a rhetorician, and has left a brief but excellent account of the assassination of Caesar, entitled "Brutus."

In Latin, now, Brutus was sufficiently trained for narrative or pleading; but in Greek he affected the brevity of the apophthegm and the Spartan, of which he sometimes gives a striking example in his letters. For instance, when he had already embarked upon the war, he wrote to the Pergamenians: "I hear that ye have given money to Dolabella; if ye gave it willingly confess that ye have wronged me; if unwillingly, prove it by giving willingly to me." Again, to the Samians: "Your counsels are paltry, your subsidies slow; what, think ye, will be the end of this?" And in another letter: "The Xanthians ignored my benefactions, and have made their country a grave for their madness; but the Patareans entrusted themselves to me, and now enjoy their freedom in all its fulness. It is in your power also to choose the decision of the Patareans or the fate of the Xanthians." Such, then, is the style of his remarkable letters.

17.5-6:

Caesar caught the handle of the dagger and cried out loudly in Latin: "Impious Casca, what doest thou?" Then Casca, addressing his brother in Greek, bade him come to his aid. And now Caesar had received many blows and was looking about and seeking to force his way through his assailants, when he saw Brutus setting upon him with drawn dagger.

40.2-4:

When supper was over, [Cassius] grasped Messala's hand warmly, and, speaking in Greek, as was his custom when he would show affection, said: "I call thee to witness, Messala, that I am in the same plight as Pompey the Great, in that I am forced to hazard the fate of my country on the issue of a single battle. With good courage, however, let us fix our waiting eyes on fortune, of whom, even though our counsels be infirm, it is not right that we should be distrustful." With these last words to him, Messala says, Cassius embraced him; and he had already invited him to supper on the following day, which was his birthday.

52.1-2:

And when Cleitus wept and made no answer, Brutus next drew Dardanus his shield-bearer and had some private conversation with him. Finally, he spoke to Volumnius himself in Greek, reminding him of their student life, and begged him to grasp his sword with him and help him drive home the blow.