I feel like we might be talking past each other but it is funny that you chose French and Swahili. [1]
The point is that "SIGN LANGUAGE" is idiomatic to the native speaker's tongue. So if you're going to take the time to create a specialized written form of it, you can just write using the native language which can be read by BOTH the Deaf and non-Deaf community.
Deaf people are not magically illiterate.
Creating a written sign language serves no value since it is just a crappier version of the normal written equivalent.
So there's not a lot of value in creating a written form of say the French Sign Language because you can just use French.
Swahili regions have multiple types of sign language including Kenyan Sign Language.
[1]
https://en.wikipedia.org/wiki/French_Sign_Language
https://en.wikipedia.org/wiki/Kenyan_Sign_Language