since Harry’s magic bag responds to sign language, can all spells be cast that way [...] ?
I very much doubt it. You have to say “prior incantato” or whatever it is, not “previous spell”, and “expecto patronum” rather than “I’m waiting for my protector”, etc., and (at least in MoR) the exact durations of the vowel sounds in “oogely boogely” are essential for conjuring glowing bats. It seems very much as if magic is keyed to particular sounds and (pseudo-)languages. There might be sign-language spells, which might make for an interesting underexploited niche for an ambitious magical researcher, but I don’t see any reason to expect any sign-language translations of spells to exist.
and (at least in MoR) the exact durations of the vowel sounds in “oogely boogely” are essential for conjuring glowing bats
In canon you have to pronounce it Wing-gar-dium Levi-o-sa and can’t cast it unless you make the ‘gar’ nice and long. (In the movies, it’s “Levi-o-sa, not levio-sah”).
I very much doubt it. You have to say “prior incantato” or whatever it is, not “previous spell”, and “expecto patronum” rather than “I’m waiting for my protector”, etc., and (at least in MoR) the exact durations of the vowel sounds in “oogely boogely” are essential for conjuring glowing bats. It seems very much as if magic is keyed to particular sounds and (pseudo-)languages. There might be sign-language spells, which might make for an interesting underexploited niche for an ambitious magical researcher, but I don’t see any reason to expect any sign-language translations of spells to exist.
In canon you have to pronounce it Wing-gar-dium Levi-o-sa and can’t cast it unless you make the ‘gar’ nice and long. (In the movies, it’s “Levi-o-sa, not levio-sah”).