The TwinBee RPG is nice for fans, but anybody else would probably be turned off by how simplistic the graphics are. It would've been awesome if they made it a super-detailed 2D RPG like how the shooters are, but no such luck. It plays kind of like Chrono Trigger since it's got the ATB system and combo attacks.
As for King's Field, yeah, I was being silly, but the stories are about as barebones as they come. They really do come down to "This world is completely fucked; save it." I've never heard of anyone playing them for the story, and I'm surprised that not understanding the tiny bit of dialogue made that much of a difference to you. I'd say the main downside to playing them in Japanese is not understanding the menus and such.