Voices and TTS in Slint.

Following TTS (Text to Speech synthesizers) are shipped in
Slint64-14.2.1.2, each with a set of voices, namely:
espeak-ng
flite
pico
mbrola
RHVoice

Most of the time these TTS and the associated voices and languages are
managed by speech-dispatcher through its so-called "modules" (roughly,
a module is associated to a TTS).

The custom utility spd-list can answer several questions about the
synthesizers, voices and languages. Typing spd-list displays this:
This script lists languages and synthesizers available for applications
relying on Speech Dispatcher, like Orca or speech-up.
Each command below answers the question following it.
Don't type the quotes surrounding the command.
"/usr/bin/spd-list" usage?
"/usr/bin/spd-list -s" available synthesizers?
"/usr/bin/spd-list -l" available languages codes?
"/usr/bin/spd-list -ls <synthesizer>" languages available for this synthesizer?
"/usr/bin/spd-list -sl <language code>" synthesizers providing voices in this language?
The language code has most often two characters, like 'en' 'es' or 'fr'

All voices listed are available in Orca and speechd-up.

You can get additional voices for flite and mbrola, associated with the
modules flite-generic and espeak-ng-mbrola-generic.

You can always know which are installed or not typing as root:
slapt-get --search mbrola-voice
or:
slapt-get --search flite-voice
then install one of the not-yet installed one, like e.g.
slapt-get -i mbrola-voice-it2

In addition to the free (as in free beer) voices shipped in Slint,
you can buy voices for:
voxin, from https://voxin.oralux.net/get.php
voxygen, sending an email to contact@hypra.fr

More voices could be made available later, this will be announced on the
Slint mailing list. 
