This is so we can have colours and other placeholders in translation keys without aggressive (and mainly Anglo-centric, or other mostly analytical languages!) splitting of existing keys.

"some.key": "<gray>Hunger:</gray> <yellow>%player:hunger%</yellow>"

The codec which encodes fallbacks for translated texts doesn't know to which player it will send translations, so I think including only the SimplifiedText format is a better idea. Other placeholders can be integrated with %s variables unlike formatting.

Fallbacks can only be strings, so we can't have text components in TranslatedTextContents