The computer code for "Sign Language" is "SGN". SignWriter files use the .SGN extension. For example, a file named "Hello", is "Hello.SGN" in SignWriter 4.4.

But what about computer codes for individual Sign Languages?

Everson Typography, working with the Deaf Action Committee For SignWriting and the Center For Sutton Movement Writing, acts as a registrar for Sign Language extensions.

When research provides positive information regarding the uniqueness of a Sign Language, the code for that language is registered with The Internet Engineering Task Force (IETF) and noted on this page:

Implementing the
ISO 639-2 code for Sign Languages