Plug in or 3rd party app to generate human speech in other languages in Adobe Audition CC 2019

The Generate Speech effect will use the text-to-speech voices installed on your system. That means it'll change depending if you're using Windows or macOS and what voices are available with each update.

macOS gives you the opportunity to download more custom voices and languages in Settings > Accessibility > Speech.

You may also be able to download custom packs from other vendors like Cepstral but I've never really played with this option.

This is the current release speaking German (actually not that bad):

Any limitations are those that Microsoft put on it, and I don't believe that there's any inherent reason for it not to work correctly if you get a natural language speaker as the voice, and put correctly phrased text into the text box. It certainly does in German, anyway - I don't know what the extent of language coverage is.

There are some commercial limitations that you have to beware of, but as far as I'm aware it works just as well as it's ever done (ie, not perfectly). Anything it won't do has nothing to do with Audition at all - it's entirely Microsoft. The only downloads are different voices - nothing other than those are available, and most of the good ones cost real money, although there's quite a selection on the Microsoft site. There is more information here that is relevant, I believe - https://support.office.com/en-gb/article/how-to-download-text-to-speech-languages-for-windows-10-d5a6b612-b3ae-423f-afa5-4f6caf1ec5d3