Just following up on this post and stomme’s comments. I heard a podcast which had a representative from the EU stating that they are intending to change European law to encompass web accessibility being a legal requirement for any commercial entity on the web. Meaning that if you sell anything or provide public services (to which you make money, even if it’s advertising revenue) you’ll be legally obligated under EU law to make your website accessible for the disabled (under commercial business practices). Some people may not like the idea but I think it’s a step in the right direction. 
It seems good to me. I’m glad I found this thread. Using the lang headers and attributes don’t seem to interfere with who can access the page in any way. And it will help make it accessible for those using a screen reader.
Makes me wonder if I should use “en” or “en-us”. Although what dialect of US English would the reader use? Regional? - Yankee, Southern, Western, etc. Ethnic? A “sterile” “news cast” tone? Mr. Roboto? Being a yankee it would be odd for me to hear my pages in a different accent but as long as it was in English I could at least still “get it”.
JAWS has a Steven Hawking voice (called “Reid”) which seems kinda accentless, and a more British sounding voice. : )
Orca has a couple of English voices, but no matter what I choose, I get the bad Britishy one (not sure which exactly it is anymore). “Weocohm ta ohca”