You do not have to make any registry changes. If you just download additional languages the OneCore voices come right along with them (provided that they are languages that support text-to-speech, some don't).
Of course, it's pretty clear that adding the language packs results in registry changes, but those are part and parcel of Windows installing those languages itself. It is not manual.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back