To be fair accents do change. You should watch earlier interviews of Mel Gibson in the early/mid 80s, he had a very detectable Australian accent. Watch any interview of him today and it’s 100% American, not a hint of an Australian accent. He has a similar background to Dern in that he was partially raised in the States then moved to Australia just before his teen years and came back to the states in his early 20s. If Dern spent 95% of her time in Brazil speaking Portuguese her accent would change. Her significant other is also Brazilian for what’s it worth so I’m guessing the accent transformation is natural not forced.