Perhaps we're not communicating well, despite the universal translator:

○ Characters cannot immediately learn sign language if they already cannot speak or write well. It's an entirely new language that requires time and patience to learn. If the character in question is going to be without their voice for an extended period of time, years even, this may be something they would want to discuss with us, as involving a third character to teach them would require that player's consent and dedicated time.

○ Characters with a minimal reading/writing ability can still make themselves understood through the translator, even if various words aren't spelled correctly.

○ Were someone who was deaf using actual sign language (American Sign Language, British Sign Language, Chinese Sign Language, etc), that language would then be translated into the sign language of another language (French Sign Language, Klingon Sign Language, etc), as that is how the translator works. There is nothing to hear as it is not a verbal language, but it would be understood through the motions, as it is a body language.

○ The character you are discussing is not deaf, they are temporarily mute.