The internal TTS engine is called "Flite TTS" (most recent, added by Torsten Dreyer), the external one is "Festival TTS" (Melchior Franz).
The process for creating/using different voices is documented separately.
How this is done in fgfs depends also on whether the corresponding hooks are added.
My suggestion would be to search the devel list archives for "Flite + voices" and "festival + voices", and maybe you could help create/update the corresponding wiki docs, too ?
https://sourceforge.net/p/flightgear/ma ... l_list=allhttps://sourceforge.net/p/flightgear/ma ... l_list=allhttps://sourceforge.net/p/flightgear/ma ... l_list=allhttps://sourceforge.net/p/flightgear/ma ... l_list=allIf in doubt, it may also be helpful to run "git log -S" using "flite" and "festival" respecively.
PS: First of all, you may want to check $FG_ROOT/Docs (README/PDF)- at least the festival stuff might be documented there. Since then, many things have admittedly changed, so that it's no longer necessarily "en vogue" among the remaining share of core developers to document new/updated features in the form of dedicated README files, but you may still be in luck.