I’ve built many language learning Android and iPhone apps over the years. I still don’t have a great formula but I think my latest iPhone apps are simple and focused enough to help get people started.
Hundred Words: https://apps.apple.com/us/app/hundred-words/id1469449237
Language Pairs: https://itunes.apple.com/us/app/language-pairs/id1438817614?...
Pictures and Words: https://itunes.apple.com/us/app/pictures-and-words/id1459560...
Word Search: https://itunes.apple.com/us/app/h4labs-word-search/id1311744...