A few clarifying questions, since sign isn't the same as a spoken language.
If one person isn't actually speaking with words, how would it come across to the other person?
Would they hear words?
Would what is being 'said' with the hands just suddenly come into their head as if the character viewing the signs (not the one 'speaking') were saying what what was being said, since there is likely no voice to be heard?
Does the above change if the character then speaks (because they can, because of a temporary injury, or with a deaf accent)?
Or vice versa, if a character is voiceless and needed to speak, would they be able to automatically sign even if they'd never learned?
Would they suddenly gain the knowledge if they saw someone else signing?
Would they then lose the knowledge when they weren't interacting with someone who knew sign language?
Is any of this different if the voicelessness is because of a temporary injury? (I.e., would they truly 'lose their voice' or would there always be a way to communicate in a way that isn't written?)
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