5:41-2 He took her by the hand and said to her, "Talitha cum," which means, "Little girl, get up!" 42 And immediately the girl got up and began to walk about (she was twelve years of age). At this they were overcome with amazement.

The second explanation is needed as few would know the age of this girl, the first explains Aramaic, presumably the readers didn't know Aramaic.

7:3-4 For the Pharisees, and all the Jews, do not eat unless they thoroughly wash their hands, thus observing the tradition of the elders; and when they come from the market place, they do not eat unless they cleanse themselves; and there are many other things which they have received in order to observe, such as the washing of cups and pitchers and copper pots.)

This would certainly tend to imply that Mark's readers were not very familiar with basic Jewish customs.

7:11but you say, 'If a man says to his father or his mother, anything of mine you might have been helped by is Corban (that is to say, given to God).

Again, this, slightly technical, point of Jewish customs seems to be unfamiliar.

7:18-19 He said to them, "Then do you also fail to understand? Do you not see that whatever goes into a person from outside cannot defile, 19 since it enters, not the heart but the stomach, and goes out into the sewer?" (Thus he declared all foods clean.)

In case you didn't catch it.

7:34 Then looking up to heaven, he sighed and said to him, "Ephphatha," that is, "Be opened."

So the readers didn't speak Aramaic?

15:21 They compelled a passer-by, who was coming in from the country, to carry his cross; it was Simon of Cyrene, the father of Alexander and Rufus.

Are the readers familiar with Alexander and Rufus?

15:22 Then they brought Jesus to the place called Golgotha (which means the place of a skull).

The audience doesn't know this non-Greek word?