Surprised noone mentions Kotlin. It's quite Swift-like, backed by JetBrains (Android Studio is based on their IntelliJ Idea), and 1.0 has only just been released. It has full interoperability with Java.
Kotlin に誰も言及していないのが驚き. Kotlin は Swift によくにており, JetBrains (AndroidStidioのベースとなるIntelliJ Idea の開発元)がバックアップしており, 1.0 がリリースされたばかりのもので Java と完全に相互連携ができる.
I love Kotlin, but being able to develop libraries in one language and use them in both Android and iOS is huge. I have been using J2Objc for this until now, and while it's a great tool, it forces me to use Java, which I don't love. I would prefer being able to use Kotlin on iOS, but using Swift for Android development is a great boon.
Kotlin emits bytecode. It is entirely interoperable with Java. All the Java APIs of the platform are accessible in Kotlin.
Swift on the other end can only target the NDK, which limits it to a very specific niche on Android.
What helps a lot is that the kotlin team has written many helper methods allowing a better flow between the android API and kotlin code : while you don't need it in order to get interop, it allows to more easily write idiomatic kotlin code while interacting with Android.
Kotlin チームの書いたたくさんのヘルパーメソッドは, Android API と Kotlin 間の流れをより良くしており, Swift ユーザがそれを利用しないことと対照的に, Androidと連携しながらより簡単に慣用的な Kotlin コードを書くことができる.
The main problem with Swift on Android is that AFAIK it is going to be limited to NDK.
There is certainly a niche where it can be useful, but for most developers, it makes it a no go.
Kotlin on the other end is indeed a very good stand-in replacement for java on Android.
No one cares if you don't use Swift. Go with Kotlin. It's probably a great choice for you. It seems like a great language and I hope it gains traction.
Swift は使わなくて良い. Kotlin でいこう. あなたにとって素晴らしい選択となる. 偉大な言語で勢いを増すことを願っている.
If all you're doing is Android programming Kotlin is probably a better choice at this time. If you're a Java programmer, Kotlin is probably a better choice. If you're already a Swift programmer...
Also, let's see how both languages gain traction in the next 3-5 years.