*Katakana. Yes, the label on the dialogue box says ナルホド instead of ナルホドー. But this is the only place where it's spelled like that off the top of my head. Your guess is as good as mine why that is - emphasizing the pun, not enough space (not in DGS, but it's been spelled that way on the dialogue box label since the first GBA game), ...
The right romanization of his name is however "Naruhodou" (or "Naruhodō" if you prefer modified Hepburn) - the "o" sound must be long.
Had you looked back a little further you'd have found this
I mostly agree with paperbag (or "alter_eigo" - I just got that pun
), but this time it's a clear case. She's right about people nitpicking about other stuff, though. There is no single "right" translation sometimes - but people who watched one or two Anime in Japanese sometimes think they're experts