[adastra22]

There’s no app that can build up conversational fluency. You can’t master a thing without actually doing the thing.

SRS apps can be good for building up a vocabulary base. Self-study drill based programs like the old FSI series can be great for internalizing grammar rules so that you don’t have to consciously think about it.

That does give you a great foundation for developing linguistic fluency though, should you relocate to the country or start immersing yourself in native media.