>>58198340
Too much letters occupying data space in ROMs which can fit in a GB cart (the whole JP version of Pokemon Red occupies circa 370 out of 512kB). Meanwhile nips (through kanas or kanjis) use 2 or more syllables per space, in comparison with the Roman alphabet which occupies only 1 character per space:
>くそ (2 characters out of 10 which can fit for a move name)
>shit (4 characters out of 10 which can fit for a move name)
TL;DR: GB ROM size too small, and any Western language uses too much characters to express things.