Because in universe,Themyscira is a secret magical island disconnected from the world of man, populated by warrior women. In addition,Diana herself isn't a human. She is a daughter of Zeus, the leader of of the Greek pantheon and the most powerful God of the said pantheon and of Hypollite, the leader of a tribe of…

In my opinion the best course would be to have Greek actresses speaking in proper Greek in Themyscira and Diana to speak English (with accent) when she is in man's world.

They are playing non English characters, namely Greek women warriors. They are meant to speak with accent because in universe they don't speak English. They speak Greek.

Diana is a Greek demi-godess, daughter of a God from the Greek pantheon, in an island full of other Greek warrior women. It is completely natural that such a person would have an accent in an English speaking country. Wonder Woman is meant to sound Mediterranean because she is. Gal Gadot's accent is fine because is…