Jumping in is really the best way to understand the contents and data structure of the ROM.
No, you don't have to disassemble the ROM in order to make changes. Sure, a disassembly, especially if one has already been commented on, will provide great insight to how the game works, but it's not absolutely necessary. Later if you're thinking about inserting some fancy script to better display the language, like VWF, then you'd want to take a look at the disassembly.
If there are no specific tools already created, you'll want to open the ROM in a Hex file editor. Translhextion will do just fine.
Specific versions of emulators that have debugging features will be what you're looking for. I don't know of any for the genesis, but I'm sure there is one out there on the Internet. The debugging emulator will help narrow down your search to a specific part of the ROM file involved with the process most of the time.
Unless you have previous experience with reverse engineering or working with these data sets, then you will surely find yourself lost. Read as much as you can to get an idea of what you can do and feel free to ask questions. We won't do your work for you, but there are others with experience who can help. There's been a lot of people who have worked with the genesis/mega drive before.