I would also like to get a solution for the same.
So far, the only way I could make this work is to use Safe Exam Browser (SEB) that has the option to disable spell check. However, SEB doesn't seem to work for question types that require audio input, hence it didn't work for me.
It would be great if this can be a configuration in Moodle itself.
Unless you are using an application that takes control of the browwer such as SEB, anyone, that is, teachers and computer savvy students, can change the spellcheck attribute since it is in the DOM.
I believe you can download this file and place it in the same place in Boost and it will work. I highlighted the one line that we needed that whole file for.