Abudarham & the angels
The medieval authority on the Siddur, David Abudarham, explains the use of Aramaic in two ways.
His first comment is that Aramaic is the language that people spoke at certain times in ancient history, and therefore when Ha Lachma Anya invites the hungry and needy to the Seder (“All who are hungry, come and eat!”), the words had to be comprehensible to the people to whom the invitation was extended.
His second theory recalls a Talmudic concept that the ministering angels do not know Aramaic, so they cannot protest against or try to undermine our declaration that our yearning for freedom is on the way to fulfilment.
This latter theory reminds us too that, according to the beginning of the Midrash Bereshit Rabba, the angels tried to stop the creation of man because he would be likely to prove unworthy of God’s blessings.