Spanish: spanish word, English word. Physical card is easy: you make one card, and go through your deck in either order so both sides are "front" and "back". The program Anki makes handling this easy: you create one note, with a Front and Back, and it generates two cards.
Japanese: Japanese word (kunyomi), English meaning, kanji, Chinese reading (onyomi) of the kanji... Never mind that you often have multiple kunyomi and onyomi. 4 items, 6 possible pairs, so 6 cards per word. Anki also makes this easier: I made a note type using just 3 fields (Japanese, English, kanji), which generates 6 cards (order matters). But that's still 6 cards to study, and it should really be 12.
Why studying Japanese is hard.
See the DW comments at https://mindstalk.dreamwidth.org/524493.html#comments