Sotah, Chapter 7, Mishnah 3. After the previous Mishnah listed the things that are said in the Holy Tongue, from here on the Mishnayos begin to detail and explain each one of them separately. The first of them is mikra bikkurim - the declaration a person recites when he brings his first fruits to the Beis HaMikdash.
"Mikra bikkurim keitzad":
The word "keitzad" - how? - in the Mishnah is not a question about the manner in which the mitzvah is performed, but a question about the source of the law: from where do we know that mikra bikkurim must be said specifically in the Holy Tongue?
The answer is derived from a gezeirah shavah between two verses:
"Ve'anisa ve'amarta lifnei Hashem Elokecha" - in the passage of bikkurim, before the first fruits are presented in the Beis HaMikdash, the declaration of mikra bikkurim is recited, and the Torah used the expression "ve'anisa" for it.
"Ve'anu haLeviyim ve'amru" - regarding the blessings and the curses that were said on Har Gerizim and Har Eival, the Torah used that very same expression.
From the identical wording - "ve'anisa," "ve'anu" - the Mishnah derives: "Mah aniyah ha'amurah lehalan bilshon hakodesh, af kan bilshon hakodesh" - just as the response stated there is in the Holy Tongue, so too here it is in the Holy Tongue. Just as the response of the Levi'im on Har Gerizim and Har Eival was said in the Holy Tongue, so too mikra bikkurim is said in the Holy Tongue.
The point that still requires explanation:
From the Mishnah itself there is no proof that the response of the Levi'im on Har Gerizim and Har Eival was in the Holy Tongue. Regarding this the Gemara says that this law too is derived from an additional gezeirah shavah - "kol" "kol" from Moshe:
Regarding the blessings and the curses it says that the Levi'im say their words "kol ram" - in a loud voice.
At the giving of the Torah it says "uMoshe ya'anenu bekol" - and Moshe would answer him with a voice - and there the words were clearly said in the Holy Tongue.
We find, then, a complete chain of derivation: from Moshe at the giving of the Torah we learned that the "kol" is in the Holy Tongue; from the "kol" we learned that the response of the Levi'im on Har Gerizim and Har Eival was in the Holy Tongue; and from the response of the Levi'im we learned that mikra bikkurim too is said in the Holy Tongue.
In summary: In this Mishnah we learned the source of the law that mikra bikkurim is said in the Holy Tongue - a gezeirah shavah of "ve'anisa" and "ve'anu" from the blessings and the curses on Har Gerizim and Har Eival, and from there, through a gezeirah shavah of "kol" and "kol" from Moshe at the giving of the Torah. The Mishnayos that follow will detail the rest of the things that are said in the Holy Tongue.