This update to Windows 10 has arrived, and it has more text-to-speech voices.
First, these are again Windows Mobile voices, not SAPI5, so your screenreader or other application may not be able to see or use them.
Second, there appear to be no additional languages, but there are reported to be “more than ten” new voices (e.g. male voices where before there were only female). I’ve identified two new accents or language variants that are new: Australian English and Canadian English.
I believe the full list of Windows Mobile voices as they stand now for Windows 10 Creators Update in 2017 is given in this Microsoft Support article: TTS voices.