I've noticed runs in both languages in the leaderboard. It looks to me like the Japanese version has shorter cutscenes between the stages. Is that true? If so, shouldn't the leaderboard be split?

The only difference in these both version is less text and nothing else. If the JP version has less enemies than US version on gold dragons or blue dragon has the boss with less HP ( just like Gold dragon) Then yes, that would be in separate category. For example Tom & Jerry. JP mode has less spikes and few other thing as US versions have lots of spike and harder. They separate that into 2 categories.

This game doesn't need separation. It just only text difference and it's less text in JP than English.