I believe the strongest evidence that Matthew was not originally in Aramaic is this verse.
Mat 27:46 ESV
(46) And about the ninth hour Jesus cried out with a loud voice, saying, “Eli, Eli, lema sabachthani?” that is, “My God, my God, why have you forsaken me?”
… Jesus was quoted lots of times. Why not quote him in Aramaic and then give the translation just as this verse does?
And if the translator was simply translating, well the original author would not have written the same Aramaic twice.
JJ
Your last comment is sound, (There shouldn’t be two repetitions of the exact same thing in the exact same language) so long as the original author of the passage was not quoting a Hebrew variant of the old testament psalm which differed in some way from the Aramaic. (Hard to tell, we don’t have those texts any more.)
The new testament reference to the present day language was to that “of the Hebrew’s” not the aramaic – because the people as a whole were known inclusively as Hebrews, the remaining sects being primarily Samaritan (outcast, interbred) and the Jews (from the southern kingdom of Judah). The parent language, however, of these diverse groups and the remnants of the Israelites was still called “Hebrew”. Just as Amercan english is still called “English” which can refer to the British english, or American english.
So, as to the first part of the answer – “why”, well the answer may well be that this reference is so explicitly Hebrew and incomprehensible to a Greek (or rather misinterpretable) that the translator wanted to make sure their audience understood it to be a foreign expression. After all, this is a victory psalm – not a psalm of defeat.
Take another twisted example used by modern scripture scholars:
In Luke there is a passage which says: “Rejoice when that day comes and dance (literally: leap or jump) for joy, then your reward will be great in heaven. This was the way their ancestors treated the prophets.”
Dance, is predicated on david before the ark – but jump speaks more to reaching up to heaven where one may eventually be if martyred.
Now many a smug stuffed shirt today will claim that “their” is a slip of the author’s pen placed on the lips of Jesus – showing that Luke is not a Jew. Although I agree that Luke is far more Greek in style – this last “modern” thought does not follow.
Read the passage a couple of times and see if you can figure out why based on Luke alone – if not, try reading: Matthew 23:2-3, 15, and esp. 29-32 which makes the accuracy of Luke’s quotation quite clear.
Sometimes the reason for things being accurate is subtle.