Long before mental illness kicked in, I had memorized 26 digits of Pi. I was extremely envious of the math tutors (who were college students, I was in HS) who knew over 100 digits.

I also studied 4 languages (plus English) way before having serious mental health issues. But that was over a period of years - so I guess time scale and intensity might be more indicative of whether that's normal vs. manic behavior.