On the way home from work today I passed a sign on a preschool that said "BILINGUAL LEARNING," and it occurred to me that "bilingual" is a pretty useless word as words come. It means that someone speaks two languages, but which two? One can generally infer that one of the two languages is the most prevalent language of the area, though even that information is not guaranteed, and we assume that of everyone in an area anyway. Compare it with "bisexual"—unlike languages, there are only two well-defined genders, and if a person is attracted to two of them, that covers the bases pretty well. Even "bidirectional" one might think at first is vague since there are clearly more than two directions, but since the word is generally only applied in situations where there are only two relevant directions (such as west or east along a street), it's not vague.
But bilingual is useless. I can make guesses as to what that second language might be, but I can't be certain. It could be Spanish, though honestly there do not appear to be a lot of Spanish-speaking people in my area. It could be Mandarin. It could be Japanese. There is a heavy concentration of Indian people near me (and this school), so it could be an Indian language, but as I understand it there are a multitude of those, so the chances of the parents of the child and the teachers at this school speaking the same Indian language would be pretty slim. These ambiguities could have been avoided with a single sign to the effect of "Teaching in English and Klingon."
So, I have concluded that the sign is nearly useless, and the word "bilingual" is entirely useless except in the case of explaining to someone how international you are.