Kotlin is here for IOS, but only with Objective-C for now. Plans are being made to accommodate Swift as well.
Converting to Swift
When converting from Objective-C to Swift theirs a massive improvement on the language. There are obvious improvements, and that’s why everybody is moving to Swift from Objective-C.
The downsides of Swift is when you upgrade from Objective-C, you have to create most of the app from scratch. Objective-C doesn’t convert well with Swift.
Converting to Kotlin
Objective-C to kotlin required barely any additional code and uses the same packages. That’s because it’s pretty much the same programming language. Kotlin has a lot of the features that Swift has, but it can be converted, and doesn’t need to be scratched.
Kotlin or Swift?
If you have time to upgrade the entire app, go with Swift. It has a ton of support over IOS and I think Apple might drop Objective-C support in the future.
If you’re looking for a quick improvement over Objective-C and don’t have a lot of time, then go with Kotlin.