I think some servers would appreciate a (serverside) config option to translate this message:

My ideas would be to either add that message to the lang files and let the server choose which one to use or to allow them to configure a custom message through the config, like plugins often do, which offers far more customization.

You could also have one config option which can be used for both:
If it is, for example, set to "en_en", the default translation is used, if no lang file with that String was found (or the format is not xx_xx), the String itself will be the message. That'll only work as long as there aren't multiple serverside messages, of course.

This suggestion is based on #45.

Servers don't have localizations without LanguageHack which I haven't updated to 1.19 yet

If it can be configured that joining the server requires installation of the mod, why is this a required feature?
They do operate differently, but ultimately achieve the same thing, unless I am missing something?

I see. The (serverside) config should still be a good and viable option, however.

@otta8634 Sorry, I missed your comment. I don't quite understand what you mean, afaik that message is shown to clients without NCR if the server does have NCR installed and requires clients to do so, too. So I'm not sure what you think is "achieving the same thing"?