I argue for scribal text.
Josephus wrote that it was generally considered "common" (lowly, base, vulgar) for a Jew to learn or speak Greek, or any other Gentile language. Even for servants (slaves) of Jews to speak Gentile languages was discouraged:
Antiquities of the Jews, Volume 20, Chapter 11
I have also taken a great deal of pains to obtain the learning of the Greeks, and understand the elements of the Greek language, although I have so long accustomed myself to speak our own tongue, that I cannot pronounce Greek with sufficient exactness; for our nation does not encourage those that learn the languages of many nations, and so adorn their discourses with the smoothness of their periods; because they look upon this sort of accomplishment as common, not only to all sorts of free-men, but to as many of the servants as please to learn them.
Many scholars have wondered how seriously to take this remark, given that Josephus himself wrote almost in the same dip of his pen how he had spent a great effort learning Greek, so that he could relate the history of the Jews to the Greeks in their language. (Although he also wrote that he spoke Aramaic for most of his life, and therefore spoke Greek with an accent.) Also, Josephus carefully documented the creation of the Septuagint translation hundreds of years before, a Greek translation of the TNK [Torah Nevi'im Ketuvim; "Tanakh"]. And archaeological finds have proven that Greek was commonly written in Judea. It seems that learning Greek, or Latin, was a practical necessity for many in that time and place.
Notwithstanding the question of how many Jews learned Greek in the Roman period, the Samaritan woman almost certainly spoke the Hebrew dialect of Aramaic (sometimes referred to simply as "Hebrew" in the gospels), not Greek. She was no merchant, nor statesman. It is implausible that she spoke Greek, or literally said, "who is called Christ" (Christos in Greek) to Yeshua [Jesus].
(Perhaps you are thinking, but the Samaritans were not, as Josephus wrote, of "our [the Jewish] nation". And that is true. But the Samaritans also viewed mainstream Pharisaism as a corruption of the true religion of Abraham and Moses. The woman even references this divide in calling attention to Gerizim in their conversation. I see no indicator that an aversion to learning Greek would not include the Samaritans also. Would someone so zealous really use a Greek term in referencing a Torah prophecy?)
So the presence of this remark in the text must be a scribal remark. It remains to ask whether the apostle or a later scribe introduced it first.
- If John's gospel was originally written in the Hebrew Aramaic dialect, as a minority of scholars believe, then the remark would most probably be scribal added later, leaving "Messiah" in the original dialect and providing its literal meaning of Christos ("anointed") when translating the text from the original Aramaic into Greek.
- If John originally wrote in Greek, or if he wrote originals in both scripts, then he could have chosen to preserve the term Messiah in the original Aramaic dialect and provide the literal Greek translation originally.
As another indicator, in my Aramaic English New Testament, which is based on the traditional Syriac text from Eastern Christanity, John 4:25 simply reads:
That woman said to him, "I know that the Mashikha [Messiah] is coming, and when he comes, he will teach us everything."
The Aramaic text does not say, "Christos, who is called Mashikha"; or, "the anointed one, who is called Mashikha"; or any such. But there are examples of Greek loanwords elsewhere in the Aramaic text. For example, Greek evangelion [gospel] is written in Aramaic Mark 1:1. So the Aramaic text indicates that the "who is called Christ" phrase is an artifact of translation from Aramaic to Greek, where the Aramaic term Mashikha is left untranslated in the dialog and translated to Greek Christos in a scribal remark.
Responding a bit to the accepted answer, I am not persuaded that the absence of the specific phrase, "which being translated", or similar, sufficiently makes the case for literal use of the word "Christos", in Greek, by a Samaritan woman who lived in the foothills of Mount Gerizim in the Herodian period. There are two common words, unrelated to this exact question, which are alternately translated from Aramaic or not in the Greek New Testament text, and when translated, it is not explicitly called out as translation:
- "Satan" seems not to be a name at all, it means "the enemy". In Luke 10:19, where "Satan" would fit grammatically as a name, we find "enemy" in the Greek text instead. So this Aramaic word is sometimes translated into Greek and sometimes not. And in the cases where Aramaic "Satan" is spelled with Greek letters instead, there is no "which means" alongside it.
- "Peter" is more like a nickname, really a pejorative, meaning "dumb". There are multiple cases of "stone" being written in Greek when not used as a nickname for Simon. But in most cases, "Peter" is not accompanied by any "which means" phrase.
When the New Testament books were first written into Greek, whether originally or in translation from an earlier Aramaic text does not matter, these Aramaic terms were written untranslated in the native Aramaic dialect within the Greek text, in certain places, and we may never know exactly why. There may be other similar cases as well.
We cannot always infer from the Greek text alone whether a potential scribal addition is actually part of the original dialogue. Comparison with the Syriac textual tradition, and historical knowledge of the culture and time period, are strong indicators that this woman did not speak any Greek.