Below Water is basically one person's labour of love - Oliver Lucanus. For the book and a lot of the video work, he works with Leandro Sousa, a Brazilian researcher, but Leandro has his own things going on.
Oliver's a good friend, so we have spoken about that local knowledge along the river. He speaks Portuguese and Spanish, but they don't speak English. It's beyond the resources of one person working alone to do translated videos with local voices and experience.
As well, I'd add that with the history of intimidation and at times murder of conservationists in the region, it might not be wise for locals to go on the record, on camera. That can be better looked at in the books and articles, where sources can get better protection (names changed for protection,etc). The government in Brazil has changed, but the economic powers are the same.