I've been successful in learning a foreign language using an app that basically consisted of reading increasingly complex stories.

I don't want to recommend the specific app I used, because it's the only one I tried, and I don't know how well it compares to other, similar apps. But there are a bunch of story-based language learning apps on App Stores. My suspicion is that most of them work relatively well, particularly compared to more typical modern language learning apps like Duolingo.

Unlike gamified apps like Duolingo, you do need to actually have the motivation to regularly use them, though. They're not going to entice you with funny animations and points and leaderboards and notifications and all of those things.