many sweets are not available in Japan, or only in restricted variations: Haribo strawberry and so on
licorice (Lakritze) (but many Japanese dislike it)
not sold in Japan: Kinderschokolade, Toffifee, Negerküsse, Hanuta, almost no mint sweets - so “After eight” might be interesting to bring, Manner waffle
chocolate: for example bags with many small chocolate pieces of different kinds, each by itself wrapped in paper. Bigger supermarkets in Germany allow you to fill bags yourself, payment is based on weight of the bag.
Kinderschokolade: seems like its not sold in Japan
Ricola Kräuterbonbons – Alpensalbei
Pullmol (Kirsche)
Dr. Oetker Vanille Pudding or Paradies Creme
Götterspeise (preparation instructions need to be translated), Brausepulver (or tablets which dissolve into drinks)