It most definitely will be a DLL and it will require a resource editor to edit. It should only be accessible to those who have some technical knowledge, understand resources and resource editors, undertand the importance of maintaining sprintf() parameters such as %s %d etc. in strings, and so on.
I don't know if I agree that tool access necessarily denotes technical knowledge. I trust that most mIRC users, scripters and tinkerers who would edit a string containing %s and %d can understand their meaning from context alone, and make those changes without further aid or fitness testing. And if they do make a mistake and cause mIRC to crash, that's how they learn, improve and grow.
Don't name the files .txt but leave them with a blank or .lang extension if you wish to discourage accidental whoopsy-doodles.