What's playable without knowing a lick of Japanese kinda depends on how much patience you have for screwing around blindly a bit until you figure things out. If you want games where you barely have to mess with text at all, your options are slim. And if you want to buy them, the more English-friendly, action-y titles tend to be the expensive ones (Zenki, Zeroigar, and Chip-chan Kick). PC-FX systems have no issues playing burned games, though, so you could always go that route (and then have access to the couple patched games). Plus, really, even if you had a copy of Zenki, the damn game is so pricey that you'd probably want to play a backup and keep the real deal safe anyway, haha. Or maybe it's just me who's paranoid.
Anyway, getting back to my original point, I'd argue that some of the kind of text-heavy games are still playable and enjoyable even without knowledge of Japanese. Like Farland Story is a strategy RPG, so obviously there are menus in Japanese and cutscenes. But it's pretty traditional as far as strategy RPGs go, so if you're familiar with the genre and don't mind fumbling through the menus until you figure out what the options do, it's easy to play. Last Imperial Prince is another game that's pretty straightforward. It's a side-scrolling adventure game a la Zelda 2. You have to wander a little to find the right people to talk to and trigger the advancement of the story, but the game does a good job of shuffling you along from area to area. Combat is a little funky, but I think it's fun.