This was discussed here a long time ago, and the post seems to suggest no consensus was reached (since there's a lot of debate on comments as well, and ultimately no answer was accepted). Further debate may have taken place in chat, because I seem to recall that in the end the decision was to make whichever title was more popular the one we use. So basically, it would mean that each case should be individually assessed.
Nevertheless, I have now created a synonym for that, and whichever language we choose for each tag, the advisable thing to do would be to create a synonym for the alternative title.
Also, if you notice the edit description when the tag on you question was changed, it says "Retagging to [parasyte] since it's better known in English by that title".
We cannot forget we're an English language community, so we should keep in mind what the best known title is for (most) English speakers.
As for you knowing and choosing which of the title is more popular, I say that's the least of our problems. If you end up creating a tag with the less popular title, either someone who is sure the other title is more popular will retag it for you; or if people are unsure, the matter will most certainly pop up in our main chat room, and a consensus will generally be reached (this has happened some times in the past).