I have two languages defined in iOS (English and French) and the native swift keyboard recognizes which language I'm swifting in.
Maybe this is supported only for specific language combinations.
It is indeed supported only for some languages - you can check the link for which ones. And it's not only about recognising multiple languages, but actual functionality that it's missing. It doesn't have a Bulgarian dictionary and support for swipe in BG, which is what I need...