I read or heard a couple of times that Arnold goes to a dialect coach to help keep his Austrian accent. He's lived in the United States for so long he's actually lost his native accent.
It happened with a friend of the family. She was born in Great Britain after the war and emigrated to California, in the early 70s, where she's lived for the last fifty years. Now, you would never know she was a British citizen, all trace of her accent is completely gone.
Same thing with my co-worker, he's Dutch by birth, raised in Wales, and lived in the Puget Sound the last thirty plus years. He's developed a flat mid-Atlantic accent.
I knew a girl in high school who had a beautifully posh English accent despite coming from Dartmouth, Massachusetts, IIRC. She said she spoke that way because she'd been raised by an English nanny, but these days, knowing how people's accents can change to fit their surroundings, I wonder if she cultivated the accent on purpose.