Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
You can change the preferred language for audio under Settings..Audio..Audio Preferred Language and the Subtitles/Closed Captioning under Settings..Accessibility..Captions Preferred Language
Caveat here is that these will only apply when "available". If a movie or series has an alternate audio or subtitle/closed-captioned option/source/track available (in this case Spanish), then Spanish will be selected. However, not all programming will have an alternate source available so will receive the default tracks of the content (presumably English).
There is plenty of Spanish content available on the Roku and Channels/apps available for install, but there is not a feature that automatically makes all content convert to another language either in audio or subtitles. The content has to include the alternative track for it to be available. Not all content contains these alternate tracks.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941