My wife is Brazilian. She was born and raised there, and moved to the US at 19. She didn't speak a word of english. She doesn't even talk like that. You can tell english isn't her first language, but she doesn't have the typical "Brazilian that mostly speaks portugese but is trying to speak english" accent. She says r's like h's sometimes, but it's actually hard to place her accent if you don't know where she's from. It could be because her english is really good...don't really know.
My point is, accents develope differently or everybody. So Dern might be faking an accent...who really cares? Does it adversely affect your life in any way? Maybe she does just speak a ton of portuguese and she's around Brazilians all the time. It's possible she picked up their accent. I don't know and I don't care.
Some of you really need a hobby.