When the plugin is used with an HTML5 theme it produces a validation error:
"Using the meta element to specify the document-wide default language is obsolete. Consider specifying the language on the root element instead."
The Content-Language meta is echoed in the qtrans_header function, but that function also outputs essential CSS tags, so I duplicated the qtrans_header function, commented out the meta line, then removed the original action and added the modified one.
I guess this could be easier if the qtrans_header function would be split, and there would be a separate action for just outputting the Content-Language meta.