Earlier quoted context omitted.
Sign language is a complex language, so something has to learn yet another language. Perhaps made all the more complex as it's spatial and visual instead of verbal or written. Not every deaf person uses sign language and there are many different sign languages in the world. American Sign Language(ASL) is but one of these languages.
> Sign language is a complex language Specifically, sign languages are not visual representations of existing languages (e.g. ASL and English) but completely different languages altogether.
I'm sure other signed languages have rough equivalents in their regional areas as well. I know Mexico has a few different signed languages, though I only have passing familiarity with 1 of their signed languages and it's definitely not a representation of Spanish.