I love your opinion. Especially the "lack of tensions" and "story wasn't dark enough". Yes, I enjoy the movie a lot even the tension is not intense enough.
For me, it's one of the best Conan Movies ever.
The 17th movie is a great one especially compared to the latest, but still, since it's a "Detective Conan movie" I don't want to get my expectation high.
Just wanna let it flow.
Of course Japanese!
I know nothing about Japanese but Conan was born in Japan and the seiyuu-s fit the voices a lot.
And anime is more interesting with Japanese.