Q:

Peter's second comment to the maiden's interrogation in Mark

In Mark 14:68 Peter denies knowing Jesus but he also says he does not understand what she meant.

But he denied it, saying, “I neither know nor understand what you are talking about.” And he went out onto the porch.
  MARK 14:68 NASB

In Matthew 26:70 Peter says he doesn't know what she is talking about.

But he denied it before them all, saying, "I do not know what you are talking about.”
  MATTHEW 26:70 NASB

While in Luke and John he only says he doesn't know him or he was not one of Jesus' followers.

But he denied it, saying, “Woman, I do not know Him.”
  LUKE 22:57 NASB
Now Simon Peter was standing and warming himself. So they said to him, “You are not also one of His disciples, are you?” He denied it, and said, “I am not.”
  JOHN 18:25 NASB

Why does Mark include this additional bit of information? Is it because as some scholars suggest Mark's gospel is a record of Peter's time with Jesus? Also, what exactly does Peter mean when he says he doesn't understand? Is it because of some language barrier since he was Galilean?

A:

The earliest ascriptions of Mark's Gospel to Mark also include the fact that his Gospel is based on his heard preaching of Peter, of whom he was the companion or secretary (and "interpreter," which might imply a job of translation).
Papias, writing in the first century (Fragments):

Mark having become the interpreter of Peter, wrote down accurately whatsoever he remembered. It was not, however, in exact order that he related the sayings or deeds of Christ. For he neither heard the Lord nor accompanied Him. But afterwards, as I said, he accompanied Peter, who accommodated his instructions to the necessities [of his hearers], but with no intention of giving a regular narrative of the Lord’s sayings. Wherefore Mark made no mistake in thus writing some things as he remembered them. For of one thing he took especial care, not to omit anything he had heard, and not to put anything fictitious into the statements.

Irenaeus, a disciple of Polycarp, who was a disciple of John the Apostle, writing circa 180 (Against Heresies, Book III, I; Book III, X):

Matthew also issued a written Gospel among the Hebrews in their own dialect, while Peter and Paul were preaching at Rome, and laying the foundations of the Church. After their departure, Mark, the disciple and interpreter of Peter, did also hand down to us in writing what had been preached by Peter. Luke also, the companion of Paul, recorded in a book the Gospel preached by him. Afterwards, John, the disciple of the Lord, who also had leaned upon His breast, did himself publish a Gospel during his residence at Ephesus in Asia.
Wherefore also Mark, the interpreter and follower of Peter, does thus commence his Gospel narrative: “The beginning of the Gospel of Jesus Christ, the Son of God; as it is written in the prophets, Behold, I send My messenger before Thy face, which shall prepare Thy way.

Clement of Alexandria, writing around 170 (Fragments):

Mark, the follower of Peter, while Peter publicly preached the Gospel at Rome before some of Caesar’s equites [knights], and adduced many testimonies to Christ, in order that thereby they might be able to commit to memory what was spoken, of what was spoken by Peter wrote entirely what is called the Gospel according to Mark.

Tertullian, Against Marcion, Book IV, V, circa 200):

The same authority of the apostolic churches will afford evidence to the other Gospels also, which we possess equally through their means, and according to their usage—I mean the Gospels of John and Matthew—whilst that which Mark published may be affirmed to be Peter’s whose interpreter Mark was. For even Luke’s form of the Gospel men usually ascribe to Paul. And it may well seem that the works which disciples publish belong to their masters.

Origen, Commentary on Matthew (circa 200):

The second [of "the four Gospels"] written was that according to Mark, who wrote it according to the instruction of Peter, who, in his General Epistle, acknowledged him as a son, saying, “The church that is in Babylon, elect together with you, saluteth you; and so doth Mark my son.”

So one would expect this to be the fuller, less truncated version of what Peter had actually said. The Greek is perhaps best translated along the lines of, "I'm neither aware of nor understand what it is you're talking about." It's simply a way of saying you know absolutely nothing about the matter and want to be left alone. It doesn't add extra information, just detail to the words of Peter, we can assume, given that the other accounts are evidently terser.
(Cf. Daniel 9:25: "Know therefore and understand" could simply be an Aramaic turn of phrase.)

