Jesus offered the Last Supper/First Mass/Sacrifice of the New Covenant using the King's English (see the KJV) so it couldn't have been Latin.
The Chaldean and Maronite Churches retain the words and language of our Lord in their Consecration. This is as close as any liturgy can come to Christ's words at the Last Supper, in his native tongue, Aramaic.
Aramaic Consecration
Byow mo how daq dom ha sho dee leh
ma' bed hy eh
nsa bel lah mo be dow qa dee sho to.
Ou ba rekh
ou qa desh
waq so
ou ya bel tal mee dow kad o mar:
Sab a khool meh neh kul khoon:
Ho no den ee tow faghro deel
day lo fy koun wah lof sagee hey
meh teq seh ou meh tee heb
lhoo so yo dhow beh was ha yeh dal 'o lam
'ol meen.
English Translation
On the day before his life-giving passion,
Jesus took bread in his holy hands.
He blessed,
sanctified,
broke,
and gave it to his disciples, saying:
Take and eat it, all of you:
This is my body
which is broken and delivered for you
and for many,
for the forgiveness of sins and eternal life.
He spoke the language of the region which was Hebrew.