Olympic had all-greek titles only on their DC-3s I think. The greek name is ΟΛΥΜΠΙΑΚΗ ΑΕΡΟΠΟΡΙΑ (Olimbiaki Aeroporia). The greek name can be seen on this DC-3:
The greek name was used on the DC-3 .The DC-4, the DC-6, the Comet 4B, the YS-11, the 707 and all following types afterwards had the englih name because the airline was becoming more and more international. I dont know if the greek name was used on the other side of the aircraft in the early days but most pictures of old aircraft featured the english one.