Trick or Treating really isn't done in Japan. Some Halloween customs are practiced, but only a few follow or know of trick or treating.
"okashi wo kurenai to itazura suru yo." means 'if you don't give me candy I will trick you", however that isn't a very polite thing to say on a stranger's doorstep that has no idea why you are there wearing a costume and brandishing a bag.
If at all it would be more commonly said as "Turiko or tureeto" (Trick or treat).
Happīharou~in
Hapi Harouin [ハピ ハロウイン]
They just use the English way of saying it. The same with Christmas and Easter.
kurai harouiin
The Japanese do not celebrate 'our' Halloween. The have their own called o-ban. If you want to write it you would use katakana (they use this writing system for foriern words) and it would be this: ハロウィーン. If you want to say Halloween in Japanese (Nihongo), you'd just have to adapt to their sounds. So you would say : HaROwiin (wiin as in lean sound wise).
As it's a foreign word that hasn't really been adopted, you just say "ハロウィン" (haroueen). But keep in mind, Halloween is basically not recognized or celebrated at all in Japan.
Say them in English. That is how you say them in Japanese. Since Trick-or-treating is a very Western thing to do. Halloween didn't exist in Japan until Westerners brought it over so "Happy Halloween" is the same in both languages; the only difference in Japanese is it is pronounced slightly differently because not all the sounds in English exist in Japanese
You can find quality Japanese Anime costumes suitable for your Halloween needs and at reasonable prices at www.partycity.com.com. The site should help with all your Halloween needs.
Halloween in Korean Chuesok
Feter Halloween.
Glædelig Halloween
'Joyeux Halloween"
Vrolijk Halloween
What do bees say on Halloween? This year I'm dressing as a zombee.
Do not open until Halloween.