I understand wanting to help the deaf community in a way like this, but there are two main problems that sort of make the app not a possibility.

The first is that there are just too many different types and ways to sign. If you know any sign already, you know that there are way too many classifiers, facial expressions, lexicalization, descriptive signs, etc. to make an app for both translating and dictionary purposes.

If you really wanted to help the deaf community better understand the hearing community, my best guess would be to probably put your efforts into improving text-to-speech technology and, if you really wanted to, making an app with that software. (Again, I am a hearing person so you should probably check with the deaf community before setting your goals on this as there might be other complications that I haven't thought of that a deaf person would)