Buying a "card" isn't illegal. It's downloading the games that is illegal. Anyways yes you can get a fully translated/patched game from the internet. Just do a quick Google search of it, should find it quickly.
Yeah it's definitely worth it to get the game. You can get through the menus pretty easily. You don't need to know Japanese at all once you learn where everything is located. Use a FAQ for all mission objectives. I didn't have any trouble unlocking everything and clearing all mission objectives.
Personally I had a lot of fun just playing by myself as well so I wouldn't worry about there not being an online community.
Gutalala Sudalala ONE PIECE, IT EXISTS! - The Great Pirate, Edward Newgate "Whitebeard"
Actually, there is an English patch that pretty much translated the whole game (only J-Quiz, in-battle word bubbles, deck maker's sorting options, deck's options and J-gems gains are not translated, and those aren't important at all). If I could, I'll join the translation team to translate the rest (except for J-Quiz, because it's pretty random), because after a few hours of playing and guessing, I pretty much knew what they mean.