Very cool thesis direction Shintaro. I like where you have gone with this. Once again my simple American mind forgets the difficulties of people in different countries or areas in which their native tongue is not spoken. Moreover, you point out that it goes beyond a simple language barrier but extends into actual illiteracy itself.
Going into this week where Steven says we are going to pursue "experimental" research methods I think it would be fun and beneficial for you to maybe setup a quick language experiment with some classmates. Maybe you could make some quick ailment flash cards (webmd.com has a good database of symptoms for diseases) and then give one student a card with the disease or ailment and have them try to communicate their illness to someone else without using and words what so ever. A charades for diseases so to speak. Like if someone had a chest pains they might clinch their chest and grimace. I think it could be a helpful exercise to see how people communicate when language isn't an option.....that and charades is fun.