There is no FREE version of Oppenheimer available, at least the recent one with Christopher Nolan. It is available through Prime if you are a subscriber. Otherwise, it is Buy/Rent from several other services.
Roku is only responsible for The Roku Channel. All others are controlled by their developers/services. In the case of Prime, language selection is not fun. You have to up-arrow from the menu bar when you start playing, then choose Language.
https://www.amazon.com/gp/help/customer/display.html?nodeId=GQ4468BXX9PLQADD