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Kotlin is the officially supported language from the platform and platform/library APIs are provided as Java APIs with optional Kotlin extensions. I don't think it's a good idea to use anything other than Kotlin for new apps. Java is best 2nd choice and easy to migrate to Kotlin.
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Flutter / Dart are not something used by Android or promoted by Android. It may be a good option for writing a cross-platform app but it's not a good option for writing a native Android only app. If resources aren't limited, native platform specific apps are going to be best.
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Users are going to use the native app written in Kotlin over an equally well made app made in Flutter / Dart because the Kotlin app will be better integrated into the platform with more familiar patterns and interfaces. Users prefer apps using the platform's toolkit / libraries.
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If you want to use the bleeding edge, more modern approach to development, that's Jetpack Compose. Heavily using Kotlin, AndroidX and the rest of what they refer to as Jetpack is the way to go for native Android apps. Flutter / Dart only if you want portability beyond Android.
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