Not a smartphone app, but desktop:
When my son was learning English (about 10 years old), I introduced him to computer game Civilization (available for free as "Freeciv" on Linux). There, different civilization advancement can have effects on your tribe (like inventing religion allows you to build a temple, which allows your city to grow bigger and still avoid unhappy citizens). So he was reading Civilopedia to know how to be better in the game.
Paid version of Civilization has 3 completely different rule sets, last one on different planet. Lots of reading! Also, tips&tricks forums provide lots of reading too. And game itself is infinite in it's variability, and good training for life (you really need to plan your resources).
Strategy/planning games with lots of rules and unit development, and with forums/wikipedias full of relevant (to the game/player ) information is just yet another example of improving the language by reading for pleasure.
Another trick which worked on my other son was public library with children encyclopedias (Eyewitness series, with lots of images) about interesting stuff like pirates, dinosaurs, romans, egyptians, war machines and stuff.
Not all games have to be first-person shooters.