hmm Hi phoenix ghankyou for stopping by.

the answer for your question is because ( Okky told me this one )

1. because of not all mute people can understand how to do the pronounciation of a word from the person who taught them. Like this you are an American , you speak a language that used by every people on earth as a Unity Language, if you american that hppen to be mute learn to talk they going to talk as a mute language of America (english) but the problem is the language of each country doesnt always seem fit with the grammar. Like so many words that use in Indonesia that cannot be translated in english.

2. If you a mute people that happen born outside of english language majority country you have translate it in 3 layers. layer 1 = sign language, layer 2 = you nattive language, layer 3 = english

3. even in somany country or nation they still doing local language instead of nation language.

so I wish that can help you out..
thankyou for stopping by again beautiful lady