To switch to Spanish, go to Settings (gear icon in the lower left), then tap Language, then choose Spanish.
iOS Gospel Library does not currently provide a way to show content side-by-side with other content (except for related content snippets, like footnotes). However, you can open the English content in one screen and the Spanish content in a different screen, and switch back and forth between them (the Screens panel is the second icon from the right in the bottom toolbar).
Samuel Bradshaw • If you desire to serve God, you are called to the work.