Flutter allows to support of Swift
programming language.
Flutter uses Dart language.
Android - Java/Kotlin , iOS - Objc/Swift could be used to write plugins and platform dependent code. More details https://flutter.io/developing-packages/
When Flutter creates new project, it generates 2 native projects (iOS and Android), that would be able to run compiled Flutter project. Languages are selected via menu you posted. Check ios
and android
folders in created project.