Of course, having Diana speak with a weaker Themysciran accent in Batman v Superman: Dawn of Justice in comparison to Wonder Woman makes all kinds of sense.
Given that Justice League and pretty much every other DCEU movie will take place in a present day setting, Wonder Woman will likely be the single film that winds up featuring the character’s heavier Themysciran vocal vibe.
That’s certainly more than enough time for her accent to soften.
The upcoming film establishes that Wonder Woman left Paradise Island in 1914, and given that Zack Snyder’s movie takes place in 2016, that means that she’s had over 100 years of experience living in the modern world when we first meet her.