Good point. Suetonius has “Chrestus” as a Latin transliteration—apparently of Christos—but he may have been confused. It’s harder to imagine a native Greek speaker making that same mistake, unless the sound change in question was dialectal.
In those days multilingualism, was very common. Anyone involved in commerce spoke Greek, if the were Roman officials they spoke Latin. Mariners from Alexandria likely spoke Coptic and those based in the port of Caesarea also spoke Aramaic and possibly Hebrew. Those from Damascus also spoke Syriac and those from Lebanon spoke Punic. It would not be uncommon for someone not fluent in a language to inscribe something in that language if it was meant to be read by many others.