WARlord1903

It's not a patch, so you can just change the messages however you want by modifying the source code. It's also open, so you can do whatever you want with it. Any item can be used as a mega stone, and the item name will be used in the message so you just need an item editor for that.

You'd need to learn ASM and most probably C to in order to develop it. Ruby is unlikely to see anyone else developing it and it would be challenging to do without adequate research, so your best bet is probably to use pokeruby.