You should know that the sounds replaced are played as a client, that means each player hears on their own side. This means that each player on the same server can hear the sound in their language, even if the addon for the language in question is not on the server.

Already I strongly recommend not publishing the default language and the other languages together, this is so that the size of the addon is as low as possible, it will be up to the player to subscribe to the language they want.

If you are here, you may have finished making a custom voice, or you want to add a translation to an addon made by another...

Supported language link [wiki.facepunch.com]
(all languages are taken into account except "pirate English")

First you need to know the path of the custom voice sounds you want to translate. For example I will have the path "npc/miku_citizen/the_sounds.wav". You will need the link above to know the name of the language you are going to translate into.
We will say that our all our sounds are already ready and stored same as in the original addon, you just have to put your sounds in a folder which has the name of the desired language, itself in a folder called "cvc" (warning, the name of the language must not have a capital letter). In my example the new path gives this "cvc/french/npc/miku_citizen/the_sounds.wav"