I find it very annoying that Google and Apple are creating new languages to solve similar problems and not worrying about cross platform. If you write for Apple you've been told to use Objective-C and Swift. If you're on Android it's Dart and now Kotlin. None of that stuff is used for Windows or Linux development or even mobile on the other OS.
I'm not a fan of proliferation of "platforms" but when it goes beyond libraries and into the language itself I consider that a very serious problem.
This is forcing developers to write apps twice, or use a 3rd party solution to run on both OSes. Yep, they're refusal to work together and create standards is allowing companies like Microsoft to have value by offering yet another solution that works on both phones.
I think you meant Java and now Kotlin? Google never recommended Dart for Android development. They have Flutter which uses Dart, but that positions itself as a cross-platform way to build apps and doesn't have any support from Android tooling or things like that. It also still positions itself as an alpha, so do with what what you will.
But Kotlin also works just fine on JVM. So Kotlin works on MacOS, Windows, Linux, and now Android. That sounds pretty cross platform to me. Kotlin is also not a language from Google, so I don't know why you're annoyed with Google on this one. Kotlin is from JetBrains.
Imagine having hundreds of languages with only trivial difference in syntax. Consider the fragmentation and effort wasted on re-inventing the same low level libraries again and again (here is the hundredth version of unit test library). Sure, only language would be bad but so is too many.
A language doesn't gain traction cause of syntax or anything like that. It gains traction because a platform developer promotes it.
C# and Java had what amounted to trivial differences, when they started.
Kotlin and Swift also aren't very far apart, both in terms of syntax and power (in Blub power continuum terms).
There's very Computer Science-y reasons for these languages to exist as separate languages, but they do cause of product differentiation and platform lock-in efforts.
I'm not a fan of proliferation of "platforms" but when it goes beyond libraries and into the language itself I consider that a very serious problem.
This is forcing developers to write apps twice, or use a 3rd party solution to run on both OSes. Yep, they're refusal to work together and create standards is allowing companies like Microsoft to have value by offering yet another solution that works on both phones.