Was the ASL-message in the American Sign Language? If so, then it's not certain that JCJ would understand it, as there is a huge difference between Swedish Sign Language and the American one, they are not similar even if both are sign languages. If he's deaf, then he might have learnt some ASL in school, the same way hearing students study English in Sweden, and he has most likely learnt reading and writing English, as English is an obligatory subject in Swedish schools.
JCJ lives is Greater Stockholm, and have been living independently for almost 40 years, so my guess is that he's able to communicate with hearing people.