How to use the gui.yml file
In the gui.yml file you can add a base64 for every language that will appear in the gui.

First of all, what is a base64?

In small words a base64 is a string containing the texture of an head.

How can I create a base64?

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 value value.

How can I add a base64 to a new language?

Just add it to the gui.yml
# Existing bases..
"your-awesome-lang": "your awesome base64"
Make sure that you have already created the language in the languages section.