Hi there!
There's actually not much manipulation going on with the actual files. RPG Maker 2003 projects don't have any way to be encrypted, so it's often as simple as having your own copy of the engine and opening the original project of OFF published by Mortis. From there, you can edit the written text directly through the events in the maps.
If you're wanting to know about the Maniacs Patch though, that's something that would take a bit longer to explain since it doesn't have a lot of english documentation. For brevity, I can only suggest you look it up, download, and install it yourself. The developer's twitter is BingShan1024, from there you can find a "getuploader" link in their bio that should take you to a repository with all the published patch versions.
I hope your translation efforts go well!