The bug

Translation still use §-style formatting instead of json text component, so custom hex color and custom fonts is impossible in translation/custom languages.

Example

You can do:

"item.minecraft.diamond": "§6§lDiamond§r"

but not:

"item.minecraft.diamond": {"text":"Diamond","bold":true,"color":"#D9A334"}

The § is outdated and stilled used only in translation and motd.