Say Japanese is my attempt to solve this problem. It's still in its infancy. It's my first time making mobile apps, so many features are missing. Btw, I also made another app called "English Speaking" for English learners.
English Speaking https://play.google.com/store/apps/details?id=english.englis...
One big problem I found so far is that it's super hard for new apps to be discovered in Google Play as there tons of language learning apps. Even Say Japanese and English Speaking provide a very different set of features (focusing on the speaking aspect), it's still difficult for them to be ranked high in search results.
My original goal was to build an app that can improve my English prosody (interword level), but I found it's technically challenging. Thus, I built these two apps as the first step.
That would be awesome if you can give me some feedback. I really want to build something that can help second language learners improve their speaking.