The languages in system.speech.synthesizer() are a bit of a mess. That is the routine used in the "Ease of Access / Text-to-Speech" settings in Control Panel, but apparently has nothing to do with any other speech capabilities in Windows, like "Narrator".
What I have found is that only some of the language packs for Windows 10, and there are boatloads, have voices supported by system.speech.synthesizer(), although they may have ones supported by Narrator. It seems that mostly the "older" and "popular" languages are supported, like English-US, English-UK, German, French, Spanish, etc.
Also, what I have found is that getting them working is a bit tricky.
What you have to do is:
1) Go to Settings / Time & Language / Language and click "+ Add a language"
2) Then on the next dialog that pops up, deselect "Install language pack and set as my Windows display language".
You want the language pack, but presumably you don't want that to be your new default language.
Then click on "Install".
3) Then click on the new language pack in the list and select "Options"
4) Then on the next dialog that pops up, DO select "Download" for the Language pack, and be sure everything in the list finishes downloading.
It will take a couple of minutes.
5) Ok. Get out of Setting....
Restart your computer. Don't shut down, RESTART.
It will install the new language pack stuff during the restart. You will see it saying "Working on Windows" or some such. This might take 10 minutes.
6) Wait for Windows to come back up. Now do nothing. Sit still for about 5-10 minutes. It does some completion of the installation after it restarts, behind the scenes. Give it time.
7) Now restart again.. I know...
8) With any luck, your brand-spanking new voice will be there. This worked for me, but as I said, only for the common, popular languages I tried. I tried Romanian, and no amount of restarting or waiting or begging or cursing would get that voice to be supported.
P.S. If you decide to "remove" a language pack, as I did with German, (auf wiedersehen Hedda!) you need to do all the same restarting and waiting and restarting and such, or the voice you removed will still be listed as available, but actually won't work. It will in fact just crash Rainmeter.
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