Is it proper English to say speak sign language?

No, it's not proper English to say "speak sign language". The correct way to say it is "sign" or "use sign language".

Here's why:

* Sign language is a visual language, not a spoken language.

* "Speak" implies using spoken words, which isn't the case with sign language.

Here are some examples of correct usage:

* "She can sign beautifully."

* "They use sign language to communicate."

* "I'm learning to sign."