In the gui.yml file you can add a base64 for every language that will appear in the gui.
In small words a base64 is a string containing the texture of an head.
With this simple tool you can generate a base64 from the head of a skin.
You have to pick the
Texture Signature value.
Here you are an easy gif that explain it.
You can also use an existing head from Minecraft-Heads, you have to pick the
Just add it to the gui.yml
base:# Existing bases.."your-awesome-lang": "your awesome base64"