I made this thread to set the rules in stone for this subject.
I know of the old debate between black/dark_magician_girl, and I do know it ended with people agreeing on using the japanese names, but the Dark Magician Girl is still tagged as such. The debate did make an appearance again when I asked to make psy_girl into esper_girl, and once again the decision was to keep the japanese name, but right now both tags are in use.
I believe the duel monsters' tags should use the official english names. I know this isn't the same as with Pokémon, my arguements are that it's kinda more like the Nintendo series tags, how the english names are much more reconizable. Plus, Aside from maybe a dozen of exceptions, every Duel Monster tags are their official english name, including the most popular and recognized one, the Dark Magician Girl.