Well, for Non-japanese people, there Katakana characters of シShi and ツTsu are hard to tell the difference. The pair of lines in the top left are near Horizontal in Shi, and near Vertical in Tsu. The curved Lines also drawn differently in Shi (up, straight) and Tsu (down, curved.) You'd know the difference if you Japanese, but not the Average 10 year old American.
Also, from a Programmers Perspective, 26 is actually easier to work out over 48. Also, you have to have all 48 taking up a Pokémon slot, and in the GBC hardware I doubt that could be possible. Also 48 formes could require an extra bit of data to cover numbers over 32 per Pokémon slot, and that could result in a smaller boxes or less boxes.
Many stuff in the Japanese market has English text on it, including the Japanese made titles. In some cases, the Japanese boxes are just a ESRB/Pegi Logo short to their American/European counterparts. (Take a look at Final Fantasy 7 though to 12, Japanese and European boxes, the only difference on the front is the Rating system and the lack of Katakana.)