Well, since Jesus read from the scriptues in th etemple, I always assumed that he was fluent in Hebrew. I forgot that the Septuagint was Greek adn was in use then. So possibly that is what he was reading and preaching from, and not a Hebrew text ?
(I also read that some time ago they found an ancient sign that when translated read "Jesus Of Nazareth's Carpentry & Discount Furniture Shop. WHOLESALE To The Public".
About four centuries separate the last book of the OT and Matthew.
In 400 years of a people of a specific nationality suffering assimilation as the Israelites did, it's easy to understand how a mother tongue could be lost and or corrupted by other languages.
We have evidence of this in our time. Consider the difference between English spoke and written 400 years ago to how it's written and spoken now. We have Indian tribes that have totally lost their original dialects their ancestors used for who knows how long since England came here in the late 1600s.
I think Greek was the popular tongue and if Hebrew existed, it existed in a corrupt way from the influences of other languages. I am sure that maybe somewhere original Hebrew was around but in small circles of Jews. Since coming out of bondage for the umpteenth time prior to Jesus's birth, the one sure way they would have had a knowledge of original Hebrew writing(not the spoken language)would have come from discovering ancient Jewish writings as we discover them today.