I am not sure exactly when this happened, but Amazon recently greatly increased the number of languages that support Enhanced Typesetting (KFX format). Internally this is being called "reflow language expansion". I do not have a full list, but it appears that most of the
40 languages supported by KDP now support ET.
Most books for sale on Amazon in these languages now show "Enhanced Typesetting: Enabled" in their description. They download in KFX format on e-ink Kindles and the iOS/Android apps running the latest versions. They are not (yet) supported with ET in the latest version of Kindle for PC.
When trying to convert books using the most recent version of Kindle Previewer (3.29.1) most of these languages fail to produce Enhanced Typesetting. An internal error message states: "Enhanced Typesetting currently supports the following languages: English, German, French, Italian, Spanish, Portuguese, Dutch, Chinese, and the Indic languages Malayalam, Tamil, Hindi, Gujarat, Marathi. Please validate that your content matches one of these languages." Hopefully the Previewer will be updated to include the new ET languages in the future. If/when that happens the KFX Output plugin for calibre should also be able to take advantage of this.