Read her creative origins. Yes she was born in Greece but she was created as an American icon, representing the American people. She was created in the 1940s to motivate women on the home front during the war. That explains why in every animation, she has an American accent. She is an ambassador for America. People always complain about sticking to the source but I'm sure y'all would complain if Supes had a weird alien accent.She wears the American flag colors because Diana Trevor (Steve Trevor's Mother) crashed on her island and died trying to save the Amazons against some monster when she wandered upon shore. They didn't know who she was or where she came from but they had her jacket which had an American Flag patch and wings so they gave her a military burial and dressed her in a suit with the American Flag colors along the wings. They made an 2nd suit for Diana when she travelled into "Man's World" because they assumed it would be recognized as the costume of a warrior.
And that was your comic book lesson for today