Unlike the episodes of the "sequel"/main series, this one is not English dubbed and doesn't even have good captions. There is a separately uploaded by fans "english subtitles" version which you should look for in case the algo did not give it to you in the sidebar. While "prequel" is technically correct, you should treat this as a fun little extra or a bonus episode and watch it at the end of the main series, just like the youtube algo/next (shift+n) button's ordering puts it. Watching it without already knowing the characters seems pointless to me and the part of the story it covers feels betterrevealed by its colorful description in the dialogue in the other episodes than by actually seeing it.