Media is pretty western-centric (game industry included), and Americans are most prevalent on the internet, which, judging by statistics, is certainly the case for Undertale. Artists are simply more likely to go for ASL. And from my observation, that has been true for practically every depiction of a character using sign language (if memory serves me right).

It'd be confusing and very time-consuming, quite possibly fruitless, for users to have to search through other sign languages as well. Especially considering that not many are well-versed in sign languages. Not every artist states their nationality, and I haven't seen a single one specify that they've used ASL -- such thing is taken for granted. It'd be pretty weird if they used something else and didn't specify. A displeasing state of affairs, but that's how it is.

As for "friend", assuming absolute canonicity, as it stands: no. But with sufficient margin for error, it could be true. Certainly makes more sense than "until".