itch.io

We have a guide coming out for translation pack creation, but to answer your question:

Translation packs are installed as folders inside the translations/ directory next to the game (the application file, so the .exe on windows)

So for someone with a Spanish and Italian translation pack, it might look like this:

• TalesOfAndrogyny.exe
• translations
  • es
    • manifest.json
    • assets
  • ita
    • manifest.json
    • assets

The manifest.json in es would contain 

{ 	"languageName": "Español"   }

and that would tell the game to add an Español option in the language dropdown.