A question related to sign language has been asked on this site.
Are sign languages treated as normal languages and are considered on-topic?
Sign language should be allowed. Though you might not speak it, it is still a language and this topic can bring up some interesting posts in the future.
Basically, sign language should be allowed as it is a language, just used with hands and usually by those who can't speak, and all normal rules on questions will apply to questions about sign language.
Absolutely, it should be allowed. Sign language linguistics research is very active. I have a close friend who is a linguist and is currently at a British university; she is going to be starting her PhD in Cambodian Sign Language (in which she did field work in developing in Cambodia).
Look up her profile at: Anastasia Bradford
Sign language is a means of communicating, other than in standard English, and should therefore be on topic here.