Allow a config to change the message sent "A Horde has spawned"
Ideally with the ability to have variations or multiple possible messages to be sent.
Good for modpacks with some form of narrative. While the default message is clear and to the point, a more atmospheric message might be preferred.

The current message is not just a string in the code it's a translatable, so it can be easily translated to different languages.

Does a lang file support multiple messages or can I only translate the msg into a single different msg?

The lang file supports only one possible message.
For several possible messages, they must be selected somehow by the mod.

Just as @mpustovoi said it, the lang files only support one message per translatable. I would have to rewrite parts of the mod to side load custom messages...