I find it strange that we have the words of Jesus talitha coum reported also in aramaic (siryiac), because there is no apparent reason to preserve the original language of exactly those words and not of the other ones, seemingly much more important from the point of view of their spiritual teaching, like those in verse 36 for example.
Mark also records the exact words of Jesus which he uttered in crucifixion :
Eloi Eloi, lama sabachthani [Mark 15:34 KJV]
Whilst Matthew reports the Hebrew of Psalm 22 (My God, my God, why hast thou forsaken me ?) which Jesus speaks in his passion, Mark reports the actual dialect in which Jesus spoke those words.
Mark refers in the opening verse of his gospel account to the words of Malachi, and he also refers to Isaiah, regarding the sending of John the Baptist as a preparatory ministry to that of Jesus. But Mark quotes Malachi first, despite Isaiah preceding Malachi and despite Isaiah being a greater prophet with a more significant ministry historically.
Mark is therefore pointing to his purpose in writing his book, namely to the Messenger of the Covenant of whom Malachi speaks.
Mark's book has similarities to the epistle to the Hebrews which speaks of the Apostle of the covenant, a title not otherwise given to Jesus himself.
So, again, in this particular incident, Mark is drawing attention to the speaker of the covenant, the actual person who is appointed to bring in a covenant that will supersede the first covenant spoken to Israel in the wilderness at Sinai. How momentous this is !
And how momentous, here, in this place, when a damsel is raised from the dead. Until here, very few have been so raised, only by Elisha in ancient times. Thus the singularity of the occasion. But it will become more common : the widow of Nain's son, Lazarus, Jesus himself, Eutychus . . .
John records Jesus' own emphasis on the importance of the speech of the Son of God in this context, the context of the raising of the dead :
Verily, verily, I say unto you, The hour is coming, and now is, when the dead shall hear the voice of the Son of God: and they that hear shall live [John 5:25 KJV]
In this he refers to something spiritual. But he expands his words almost immediately :
Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice, [John 5:28 KJV]
The very voice of this One shall raise from a spiritual death to a new Life.
And the very voice of this One shall raise the dead in resurrection, globally.
He is the Messenger of the Covenant, sent to speak that New Testament.
And he is, as Malachi reports :
Behold, I will send my messenger, and he shall prepare the way before me: and the Lord, whom ye seek, shall suddenly come to his temple, even the messenger of the covenant, [Malachi 3:1 KJV]
. . . the Lord, himself.
I would suggest that the Aramaic original words of Jesus were reported here because of kind of play of words linking up the two miracles narrated in Mark 5 and Matthew 9. In Matthew 9:20 the woman healed from her persistent bleeding (12 years) is in fact expressedly told to have thought of the fringes (kraspedon which translates tzitzit in LXX Greek) of Jesus' mantle as what to grab on to get healed, which makes us recognize it as the prayer shawl the Jews still call tallit. Now, tallit is a non biblical word of probable Aramaic origin which in Hebrew is still written with the same consonants as the Aramaic word Jesus used to address the girl: Talitha. The meaning of the fringes is quite clearly expressed by the Hebrew and Greek names of the borders of the mantle, which they had to be attached to (Numbers 15:38-39). Referring to the tallit spiritual significance, Jesus would be explaining the spiritual action he was performing upon those who were looking for shelter under his wings (cf Mat 23:37: "O Yerushalayim, Yerushalayim, you who kill the prophets and stone them who are sent to you, how often would I have gathered your children together, even as a hen gathers her chickens under her wings, and you would not!"). It may be only a coincidence, but reporting the use of almost the same Aramaic syllabes as the ones that refer to the Aramaic name of prayer mantle could be an additional way for Mark to notice a meaningful link between the two miracles of chapter 5 (beside the number of years of the woman's sickness and the girl's age 12 in both cases), and also to show in another way that Jesus' ministry is the LORD' completion of the Old Covenant of the Law and the Prophets unto the New Covenant of the Kingdom (the Law written in our hearts, when we spontaneously want to have the will of God done in our life and actively ask for it).