I wonder how many native speakers of a sign language learn a second sign language. At a hunch, I’d imagine that fewer do than those who use a vocal language, if only because foreign languages are compulsory in many schools I know (which all use speech for instruction) but I don’t know whether the same is true for schools taught through the medium of sign language.
I also wonder what the situation is for readers of Braille: how many of them learn the Braille system of a foreign language. Here, I can much more easily imagine such a person learning a foreign (vocal) language at school, but I don’t know whether they would get taught the Brailly system used by native speakers of that language.
For example, would a German person learning English at school use German Braille to represent it, English Grade 1 Braille (which, I think, differs mostly only in a couple of punctuation marks from German basic Braille), or would they learn “proper” Grade 2 Braille (which, of course, has all sorts of different abbreviation rules compared to German contracted Braille)?