What is the new feature or improvement?

[[groups]]
color = "#d4781c"
decay_multiplier = 1.0
beneficial = true
name = "groups.diet.fire.name" <---here
icon = "minecraft:cooked_beef"
gain_multiplier = 1.0
default_value = 0.0
order = 0

Sadly when I did that the game crashes and I get the error Tag diet:fire used before it was bound

No, it seems you're trying to use the whole language key as the name, which is incorrect. It only needs to be name = "fire", if you wanted your group's identifier to be "fire".

It's because you're creating a food group with no foods. Add some food items to the diet:fire item tag and that error will go away.

Omg I did not add the "" xD Thanks <3