language penpal sites.
It's not an app... but rather learning with others.
Advanced-level language learners may want to transition into actual real live conversations-- it's so you can spontaneously learn new concepts, expressions, and vocabulary.
I used to use a good penpal-language learning website called MyHappyPlanet. It was back around 2007-2010-ish, and the site no longer exists.
But looks like there are alternatives here: