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"item.minecraft.diamond": {"text":"Diamond","bold":true,"color":"#D9A334"}The § is outdated and stilled used only in translation and motd.
Confirmed for 1.19.3; it shouldn't be too hard to detect a "translate" loop and invalidate the file if that's the case.
(To clarify, confirmed the alternate option isn't implemented.)
This probably is considered a feature request though.