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That is correct
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Morse code. Semaphore. International maritime flags. Literally anything displayed on a screen or written on paper. >

If you're gonna use a medium, then all languages work. Radios, paper ect. >

Are your hands not a medium for sign language
You could write on your hands >

And air a medium for voice >

There's no air in the Vacuum of space >


if you’re gonna use a medium  

That’s my point, if you change it to allow mediums like the top level comment said anything could work including voice through air >

But we are talking about mediums for languages that you can use in the Vacuum of Space so your point is moot.
P.S.
Radio waves can propagate through Vacuum which is why it was mentioned as an example. >

This was one of my favorite details in The Expanse.  Belters, who spend a lot of time out in space, have a lot of common gestures for situations where you're out in vacuum, they have signals for things like "no comms," "do not approach," "explosion hazard," and such. 
Even when they're inside, none of their communication gestures involved your hands/arms being in front of your torso.  Their gestures are bigger, with arms out and away from the torso for better visibility.  There's tons of lines in the books like "Naomi gave a Belter shrug..." which isn't just a shoulder movement like Earth- or Mars-born people usually do, but involves both arms being extended out and raised in a shrug-like motion. >

And, if I recall, they don't nod/shake their heads, but rather their fists, held up/out near the helmet