It's common for anime to be produced in Japanese first, as that's where the original source material (in this case, the manga) comes from. The English dub is a translation and adaptation of the original Japanese content.
It's also worth noting that the English dub of Sailor Stars is often criticized for its changes to the original storyline and characterizations, as well as its inconsistent voice acting. Many fans prefer the original Japanese version, with subtitles, as it offers a more authentic experience.