Now, a couple critical points are in order. Although writing in the 4th-century Eusebius is here quoting from the work of Papias of Hierapolis, from the early-2nd. The fact that Papias's reports come to us via quotation is not particularly problematic, at least not for those who understand the conditions under which historians of the ancient world work: a great deal of our data comes to us this way, and if we were to ignore all such data our knowledge of the ancient world would be greatly impoverished. For instance: pretty much all of our knowledge about Greek philosophy before Plato (including that of such figures as Pythagoras and Socrates) comes from material quoted by Plato himself and subsequent writers. Likewise with the fact that Papias is quoting from "The Presbyter" (or "Elder"--perhaps the Elder who self-identifies as the author of 2 and 3 John): again, absent such preserved oral tradition we would know nothing about, for instance, Socrates. When these basic critical matters are squared way Papias becomes our earliest extant extra-canonical, direct, reference to the production of the gospels: not a set of data to be lightly ignored."And the Presbyter used to say this, ‘Mark became Peter’s interpreter and wrote accurately all that he remembered, not, indeed, in order, of the things said or done by the Lord. For he had not heard the Lord, nor had he followed him, but later on, as I said, followed Peter, who used to give teaching as necessity demanded but not making, as it were, an arrangement of the Lord’s oracles, so that Mark did nothing wrong in thus writing down single points as he remembered them. For to one thing he gave attention, to leave out nothing of what he had heard and to make no false statements in them.’”  This is related by Papias about Mark, and about Matthew this was said, “Matthew collected the oracles in the Hebrew language, and each interpreted them as best he could.”
Thus far the critical niceties. Let us begin with the analysis.
It is interesting to me that Papias envisions Mark's work as well as Matthew's in turns of translation. "Mark became Peter's interpreter" could just as easily read "Mark became Peter's translator," and in any case what we are dealing with here is the idea of rendering Peter's teaching about Jesus intelligible within the horizons of others. The same idea is present with regard to Matthew, and the other horizon is referenced as the Ἑβραΐς διάλεκτος: the Hebrew "language" or "dialect." After some years of struggling on the question I have become persuaded by Francis Watson's recent work that the Ἑβραΐς διάλεκτος is best read as a reference to a "Hebraized" Greek, and that Papias is referring here to the Gospel of Matthew itself, not to an earlier Aramaic or Hebrew Gospel. But no matter: what matters is that Papias envisions Mark and Matthew undertaking the work of interhorizontal communication.
"Interhorizontal communication" is a term of my coinage to reference communication between two horizons. "Horizons" is Lonergan's term, and denotes the things that we care and know about. It includes culture, language, etc.: in other words, the conceptual resources that we have near to hand through which to construct and discuss reality. Ben Meyer uses this concept to great effect in Early Christians, arguing that earliest Christianity was defined by the need to negotiate the differences between the Hebraioi and Hellēnistai, known to us from Acts 6:1. There is good reason to think that Luke is being overly schematic in presenting these two groups, and that in fact the church from the off evinced even greater horizontal diversity than is encompassed by these terms (and that Luke knows this: cf. the Passover account in Acts 2), but the passage is sufficient to demonstrate a retrospective awareness of such diversity in the earliest period of the church. Such diversity marks the entirety of the New Testament: what are Paul and others doing but to figure out how to get Christians of diverse backgrounds to coexist in unity? Interhorizontal communication is at the heart of the epistles, and Papias seems to envision that it lies at the heart of the gospels also.
This is hard to reconcile with the habit of supposing that each gospel represents the thought of a single community, or perhaps a single trajectory, within early Christianity. Such habit treats the gospels as each functionally a horizon on to themselves. Instead, what is suggested here is that we view the gospels and much of what Paul, James, etc., are trying to do as efforts to transcend horizons, to fashion a unity that stands above but not against diversity. That is, Mark, Matthew, and potentially Luke and John, should be read less as instances of difference and more as concerted efforts to negotiate difference: a not insignificant shift in our perspective.