[edited by: SEMblahblah at 9:35 am (utc) on July 25, 2008]
They may still read the majority of pages they visit in English, but if they want to send an email home or read sites in their native language, they'll want to be able to see non-English characters, usually by toggling a shortcut on the desktop.
In fact Asian and Greek friends who have lived in the USA for decades showed me how they have these different language sets set to non-English even as they read pages in English.
Do you want to lose those visitors?
I believe Google frowns on the practice of advertisers setting AdWords for languages their sites don't support, but it may not be enforced.
I tried to eliminate one language from my list that always read "All lanaguages" and now I'm unable to put it back. Anyone else experience that.
All JMHO, corrections welcome!
Israel