Each story opens in a language combination. By default the learning language is Spanish and the native language is English. Change this in the settings to match your needs.
On the detail page of a story, it's possible to open a story in a another language not selected in preferences.
Pick one of the stories from the library and open it. By default that will open with the pace reader. That allows you to read the story with a focus on an individual paragraph.
Use the up or down buttons to move to the next paragraph of the story. The keyboard buttons for up or down will also work.
Either click or tab the toggle button to switch to the second language. Use the t key to quickly switch between languages on the keyboard.