Kotlin is the other 20%. It doesn't have to be. You can instrument the JVM from Haskell via JNI for all the JNI stuff like the UI, only needing a few shims.
The app could be 99% Haskell if it wanted to.
GitHub
inline-java/jni at master · tweag/inline-java
Haskell/Java interop via inline Java code in Haskell modules. - tweag/inline-java