If client has essential commands installed, use normal translatable text, so they get feedback in their preferred language.

If client does not have EC installed, send LiteralText of server's selected language to the client.

You could use ServerTranslationAPI to handle it?

Ooh, that sounds distinctly useful. Thanks for the recommendation! If you've got an example of a mod that is already using this (or docs) that I can use to get up to speed, that'd be greatly appreciated. No pressure if not. I do genuinely appreciate you putting this on my radar.

Most Patbox mods use it, Nucleoid too.

Also have to do versions detection or check translations on client... otherwise, if they have an old vers, the text will just be the translation key. Pain.