A hex editor is not a good tool for ROM hacking. They can do small changes and might help you figure out a format but if it gets beyond the point where you can keep everything you have to do in your head then you are using the wrong tool.
The Persona series might have some tools available for it as it is rather popular. That said if you did not find any during your search then they probably don't exist (yet) -- there tend not to be hidden tools known only to a handful of people in ROM hacking as most things get spread around quite well.
I should also ask what language? Depending upon what you want to go to then you may need some additional hacks; other Asian languages, middle Eastern ones and even some European ones need more work than you are likely to see a tool for.