I feel like Aura did adapt somewhat, he probably understands a lot more Portuguese than the documentary lets on. However, watching this I feel like they both had a severe mental break caused by whatever they experienced which may explain why he keeps repeating the same story.

I tried to imagine myself in the same situation: surviving (his equivalent of) an apocalyptic event, getting picked up by the people/beings that look strange and don't understand anything I say (and who are likely the ones who triggered said event), the only point of contact I had dying, having to learn to trust people I don't understand, knowing I'm the last of my people... I wouldn't be right in the head either.