[tomsem]

You would not know from reading Version history, but the feature is available in the current Kindle apps for iOS/macOS and Android (in Aa|More options). (Subject to DRM restrictions of course)

It supports multiple languages (at least English, French, and Spanish, but not Russian), and the voice data seems to be part of app resources, as there is no visible way to choose or download or remove voices.

I noticed the new terminology and implementation (highlights text, and reading speed - TTS or audiobook companion - can be adjusted by 0.1x increments) on FireOS Kindle app a couple of weeks ago; for all I know it was already there also for the other platforms.

Windows seems to be left behind, again: the Android app does not have this feature, even though it is same version found on actual Android devices. And I doubt the native Windows app has it either (have yet to check that yet).