This is a resource to allow developers to easily add translations with little work on their end. I am aware that there’s a pull request to add translation support into sponge itself via #1224 of the Sponge Api but I don’t know when that will get completed/merged. I made this to use in the meantime while we wait for it.
For developers wanting to use this in their plugin, the documentation (as of writing, only basic usage is done. Will complete more later tonight) should help you.
As a server owner using plugins supporting this, all you have to do is download it and install it onto your server like a regular plugin.