app is very broad, even web applications could be considered apps. Assuming you mean mobile apps, there is kivy. Never worked with kivy, but there website says we can make mobile (touch) apps.
For my Android Apps on mobile devices I use Kotlin. It is accesses the Java base well. Codecademy has a great beginning course on Kotlin too.
Here is a link to there brand new page for Kotlin. https://www.codecademy.com/catalog/language/kotlin