Sotah, Chapter Seven, Mishnah Three
This mishnah introduces the midrash which proves that the declaration made upon bringing the first fruits must be in Hebrew.
How is it that the declaration made [at the bringing] of the first-fruits [must be in Hebrew]?
[It is said], And you shall answer and say before the Lord your God (Deuteronomy 26:5), and elsewhere it is said, And the Levites shall answer and say (Deuteronomy 27:14); just as the answer made elsewhere must be in the holy tongue, so must the [declaration discussed here] be in the holy tongue.
That the declaration made upon bringing the first fruits must be in Hebrew is derived by comparing the word answer in the passage concerning first fruits, with the word answer in reference to the blessings and curses that the Levites give to the children of Israel after they have entered the land. Just as the latter must be in Hebrew so too must the former.
However, the mishnah does not explain why the blessings and curses made by the Levites must be in Hebrew. The Talmud explains this as a midrashic connection between the word voice in this verse (Deut. 27:14), with the word voice in Exodus 19:19, As Moses spoke, God answered him in a voice. [JPS translates thunder but the word kol is the same in both contexts.]