Text to speech with W10: non-standard languages

Anki’s text-to-speech capabilities look mighty cool, but I want to make use of them with a less widely used language, and I can’t figure out how to do that. Using {{tts-voices:}}, I get the usual suspects:

TTS voices available:`
{{tts de_DE voices=Microsoft_Hedda}}
{{tts en_GB voices=Microsoft_Hazel}}
{{tts en_US voices=Microsoft_David}}
{{tts en_US voices=Microsoft_Zira}}
{{tts es_ES voices=Microsoft_Helena}}
{{tts fr_FR voices=Microsoft_Hortense}}

But I have other languages installed, and they show up quite nicely in Settings|Time & Language|Speech|Voices. Is there a reason {{tts-voices:}} doesn’t pick them up? Is there a way that I can force their use? It would be awfully nice to be able to use them to get a native speaker pronunciation of vocabulary. Or is this a pipe dream?

Please see https://github.com/ankitects/help-wanted/issues/16