We have been fighting this problem for a long time. The 'cures' offered are to a large extent valid - but not the whole story.
Bad news. Not only do you have to insure your Page settings ( Tools - Site Settings - Default Page Encoding ) shows Unicode UTF 8 - etc.
but we've also found - any existing pages with the dreaded < span en-us > will have to be manually removed from the HTML.
It seems that if FrontPage sees the encoding between, in, or near :-) any Paragraph or Table, etc. it will follow in kind and continue to add Span Language between anything you do.
In our 'crazy-making' search for a 'fix' - we discoverd that on the same page - some areas would add the code and another area would not.
It just didn't make sense! - until we looked closely at the HTML in the associated vicinities. Where any < span en-us > codes in any form were present already - FP would continue to add more with any changes made. No pre-existing < span en-us > in the work paragraph, table, background setting, etc. - FP would not add the dreaded Span Language.
This holds true regardless of the afformentioned setting checks and MS Office Updates.
Not sure how Expression Web handles this issue - at present. After a few months of using it - we decided the general 'frustration' factor with Expression Web just wasn't worth any new Bells & Whistles it might have. At least not for our current needs.
Back to manually removing thousands of those insane Span Language tags. Argggg!