And what about screen readers for visual impaired users?
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<link rel="alternate" href="http://example.com/de" hreflang="de" />
<link rel="alternate" href="http://example.com/bg" hreflang="bg" />
<link rel="alternate" href="http://example.com/en" hreflang="en" />
From my knowledge with a very close Blind friend who uses a Mac for his screen reader, the iOS and OS X is very good at reading text on a website. As for images, you can forget it. The program will tell him there is an image on it, but it can not read text or what the image is about.
Also, he and a lot of people scrap a webpage for the reading of text only who is blind.