Kotlin is well established in mobile as a language for Android development, if not the language for Android development.
But to be mobile development tool in general, kotlin needs to also work for iOS. The path to iOS applications is Kotlin Native
Kotlin Native was first released on April 1st 2017. It was an actual early access preview and not an April fools joke, but did not at that time include iOS as a target. In fact pieces of iOS support gradually started to appear by August, but see the reviews below for what it could do at which date.