Google is launching a sign language interpreter app and keyboard shortcuts tied to its video chat called Hangout. The Hangout Captions app, for example, enables live transcription services for the deaf and hard of hearing. It allows you to invite interpreters to speak and sign for you during a Hangout. You'll see your interpreter at the top right of the window and become the focus of the Hangout whenever their interpreter is speaking for you in sign language. Check it out here or you can directly install the app using this link.
Then there's something to make it easier for sign language speakers to do a Hangout together. Look at the new Take the Floor here keyboard shortcuts. If can't or don't want to use a mouse during the video chat, you can use these shortcuts. To view the full list of keyboard shortcuts just type '?' while in a Hangout, or visit this page. Here's a video about Take the Floor which allows you to "take the floor" in a hangout with no mic.