please see #5516 for motivation on this, but I don't think anyone will be complaining - this behaviour made no sense and caused a lot of confusion for people who were trying to set custom ban messages.
this allows localizing disconnection screens (at least, once #4512 has been addressed) and the disconnect reasons shown on the console.
We already had disconnect messages implicitly localized in a few places, so this is just formalizing it.
This does break BC with any code that previously passed translation keys as the disconnect screen message, because they'll no longer be translated (only Translatables will be translatated now).
