So I've recently come up with a bad idea that I would really like to have. I want everything to replaced with runelic by default- basically treating it as a language setting if possible. I don't even know how difficult this would be to implement, but I would love to play with it.

Unfortunately there is nothing built in to allow this, Minecraft's system just isn't set up to allow this. There are some ways to hack it in, but I am not too keen on doing that.