But “It’s Greek to me” goes right back to the Romans.
The wiki seems to say the aphorism originates with medieval scribes and Shakespeare’s Julius Caesar.
The actual ancient Romans are unlikely to have had such qualms, since at the time Greek was much more widely understood than Latin, so much so that many important roman works like Caesar’s Memoirs and Marcus Aurelius’ Meditations were originally written in Greek, with the Latin versions being translations.
Pedantic note: Yes, Meditations (a phisosophical treatise) was written in Koine, Commentarii de Bello Gallico (veni, vedi, vici—self-aggrandizing combat-reports meant for the senate and propaganda) or other “published” works from Caesar were not.
Although bonus points, the ancient sources portray Caesar (a proper educated major family Patrician) as speaking his dying words—if reported saying anything at all—in Greek, not in Latin: “Καὶ σὺ τέκνον” (Even you, child) rendered in Shakespeare as “Et tu, Brute”.