I dunno, the furthest I got in formal language instruction was tenth grade English. No, wait! It was fourth-semester French. (It's odd to hear you say that Japanese has weakened your grammar, when foreign-language study is the way most of the people I know learned the few rules we do know. But I guess Japanese is just Special like that.) If I could re-do college, I would definitely find the time for an intro-level linguistics course and at least one intro-level macroeconomics course.