We haven't made an official documentation, so this post will make do:
Right now, there are two "talking" services, which are Pushback and Deicing, that have voices located in the following folders:
Addon Manager\Couatl\GSX\sounds\pushback
Addon Manager\Couatl\GSX\sounds\deicing
The "root" of these folders represent the "default" English-American voice, and inside there is a variable number of sub folders named as the language code for that country, which are associated so a list of countries, which are in turn associated to the ICAO code of the airport in use.
So, for example, let's assume you want to create a Portuguese voice set (your choice if you want to speak Portuguese or English with a Portuguese accent, which might be more correct ), the language code for Portuguese is "pt", so you'll have to create the following folders:
Addon Manager\Couatl\GSX\sounds\pushback\pt
Addon Manager\Couatl\GSX\sounds\deicing\pt
I suggest to simply COPY all the WAV files from another language variation in the "pt" folders, for example "en-gb", so you can simply overwrite the files with your variation, while you are doing them.
Once you created the files, they will now play in the following countries ( first two letters ICAO matching ), because they have been assigned the "pt" language code.
"FN","Angola"
"FQ","Mozambique"
"GG","Guinea-Bissau"
"GV","Cape Verde"
"LP","Portugal (including the Azores and Madeira)"
"PT","Federated States of Micronesia, Palau"
"SB","Brazil (also SD, SI, SJ, SN, SS and SW)"
"WP","Timor-Leste
Find attached a list of all countries and their associated ICAO and language codes, taken directly from the GSX source code.
Also, I suggest to create an empty DONTOUCH file in each of your custom voice folders, which is used to tell the FSDT Live Updater to not change or remove those folders.