In the movie, Hanks's Parker has a thick accent that is meant to "other" him and make him seem suspicious. Nash told Variety that Luhrmann "thought it was very important that Tom present the audience with a strangeness, a sort of 'What is going on with this guy?'"
In reality, Nash says Parker had an accent that was American rural, with what people thought to be a speech impediment. It turned out it wasn't a speech impediment but rather his attempt "to wrap a Dutch tongue around the English language, Southern style".