I've noted that Jesus probably spoke little Greek and did not have formal training. Similarly, Peter and John were likely not trained in Greek (see Acts 4:13). Bart Ehrman points out:
[The four Gospels of the New Testament] are written in Greek, by highly educated and well-trained authors, some thirty to sixty years after Jesus’ death. Jesus’ followers, however, were Aramaic-speaking peasants from Galilee who evidently did not speak Greek, let alone know how to compose lengthy accounts (or even to read) in Greek. The Gospels of the New Testament were apparently written not by his closest followers in his own day but decades later by more highly educated Christians who based their narratives on oral traditions that had been in circulation in the intervening years since his death.—Truth and Fiction in The Da Vinci Code, pp.107-8
Arguably, Mark was the earliest attempt to document the life of Jesus, but the text is Greek and not Aramaic. That implies that the author of Mark was not a first-generation or even a second-generation follower of Jesus, but a later follower who was proficient in Greek.
Contrary to Dr. Ehrman, can it be argued that Mark was actually an Aramaic speaker who acquired Greek later in life? Could he, in fact, have learned Greek in order to be a witness or transmit oral traditions to Gentile converts?