You don't have to login it is not mandatory.
You have to be 13 years or older to sign up for an account. You also need to have a Facebook account to sign up.
It is perfectly safe to sign up for WikiAnswers.
You just press sign out.
No, a Wikianswers account is free of charge.
You have to sign up for it.
You can. You have to create a user name and password though, just like an email account. Then you can sign in!
When you sign up for WikiAnswers, you are asked to choose your own username and password; you can personalize it from there during the sign-up process.(See the Related question on how to become a member of WikiAnswers.)
You simply need to sign-in (create a username by clicking register or one of the "sign-in" tabs) and then click "edit links" in the right column.
You do now ! You need to register using Facebook (that wasn't the case when I joined Answers back in 2009).
Sign language is a visual language. However, WikiAnswers is a written medium, we cannot convey a visual answer to your question. Also there are various different sign languages and you would need to be sure that you learn the one relevant to your country.
You can't
No, You cannot log on to Facebook from WikiAnswers. You can sign into WikiAnswers using your Facebook account however.