A suggestion taken from SEOquake Diagnosis tool is to add a lang attribute in HTML tag. I've done it in my custom theme by replacing
` with

Actually you can also set the language via the ``tags in the head, which the html_meta_tags plugin can do if setup for it.

Thanks acrylian, I missed it already was in that plugin...

EDIT:
By the way, for some reasons SEOquake Diagnosis still reports Language: Undetected after I've enabled the plugin, enabled both the language meta in the plugin options, and removed the lang attribute described in my first post.

Sorry, I really don't know that tool but the meta tag way is for sure standard procedure. Be sure to clear the html cache if you use it.

I don't know how reliable is that tool either, btw I have html cache disabled and I can't even use that, because of a problem with custom page as gallery index, very similar to the one we found about sitemap generation (http://www.zenphoto.org/trac/ticket/2160). But now I'm off topic. I should open a ticket about that.
I see now that you have fixed the sitemap issue, time for me to update again...