sadly, there seem to be pretty few options here.

as noted above, there is MS-SAPI and Festival.

there is also FreeTTS, which is written in Java.

there are also several GPL TTS libraries.

as for using SAPI, it comes with a few drawbacks:

using it is not portable to non-Windows targets;

one is more or less at the mercy of the OS and what software the user has installed as to which voices are available.

in my case I wrote my own, but it mostly uses speech data originally scavenged from Festival (and converted into custom formats).

I have not had as much luck with various attempts at creating synthetic voices, and doing the extensive recording and similar needed to make new custom voices is problematic, ...

so, yeah, data is probably the harder part here.

to some extent, in my case, it is possible to post-process the audio to change pitch and a few other things (at the cost of a reduction in terms of audio quality and intelligibility), as well as cause generate speech emitted from various in-world entities, ...

it is not particularly good, but it basically works.