The short answer: Most likely, Mark translated the Aramaic in 5:41, 15:22, and 7:34 for the benefit of his Roman readers, some or most of whom may not have read Aramaic. Many Roman citizens could speak Aramaic, particularly traders, shippers, bankers, vendors and the like, but not every Roman could speak it, let alone read and write it.
Another answer could be that Mark liked the sound of the command. Perhaps he himself had grown up hearing his mother's mellifluous voice saying the very same thing to his sister when it was time to get up for breakfast and get ready for school--bet sefer,בית ספר. In a combination of Aramaic and Hebrew she may have said:
"Talitha koum, Sara. Bet sefer!",
which being translated from Aramaic and Hebrew means, "Get up little girl. School!" Sara, by the way, attended Yeshivot Simeon ben Shetach, an extension school located in Mark's hometown of Cyrene in Pentapolis--modern day Libya.
As for Mark 7:34, where Jesus looked up to heaven, sighed deeply, and said to the mute man whom people brought to him for healing, "'Ephphatha! that is 'Be opened!'", again we can only speculate.
Was the word included for the benefit of Mark's Roman audience?
Did the word have special significance to John Mark for some reason about which we are unaware?
Did Mark's witnessing the event etch the word in his mind in such a way that it simply "slipped" out of his mouth unconsciously and onto the papyrus, since Aramaic was likely his first language?
Did Mark simply like the sound of the command in Aramaic for some unknown reason?
Each of the above questions, if answered, could provide us with an acceptable answer, at least in theory.
In conclusion, as Marianne Dorman observes: "The Marcan Gospel was written by someone who knew Greek but not very well as this Gospel is written in poor Greek, especially when compared to the Lucan. It would seem we have an author who thought in one language, probably Aramaic but had to write his thoughts in another in Greek as the community to which he is writing did not know Aramaic"