My recollection is that it's legal to make a backup of your own software. This means copying your own cartridge, not downloading a copy of someone else's cartridge. I thought about copying my Super Metroid cart when I was having trouble with it deleting saves, but in the end it wasn't necessary. I don't think using the backup has to do with the first one breaking, but it can only be a single copy of your specific original.
The catch is, I suppose, not too many people have cartridge copiers, so it's hard to be strictly legal.
Wee! The legality of ROMs is such a fun issue. As was said earier, it is only legal to make a copy of your OWN game, and only for back-up purposes... you aren't actually allowed to use the back-up. However, PD games (Public Domain) are completely legal to download and play... because they were made by fans and those people declared them to be free to all. What game are you wanting to translate? I should be able to help.
If your ROM is zipped, unzip it and then place the patch in the same directory as the rom. That should be all that you need to do with that. Oh, and make sure they share the same exact name.
It seems I'm a little late on this one, but I'll talk anyway. Downloading ROMs is never legal, unless they're public domain. You'll see on ROM sites notices that you have to delete them within 24 hours, otherwise they're illegal... that's bull. They're illegal, period. They'll also tell you it's okay as long as you own the game. As stated above, that's still illegal. Unless you made the ROM yourself from your own game, or unless it's a public domain fangame, there's no way you can own a legal ROM, end of story.
I was an exclusive GB/GBC/GBA gamer for awhile, and I'm certain that the back of all the instruction booklets said that backup copies of the cartridges are not necessary and not legal. I don't know what they were talking about with the "not necessary", though, because those grey cartridges were fragile. I've had random memory erasure problems with GBC cartridges, even.