My site language is undetected?
I have this in my html.tpl.php file:

&lt;html
    xmlns="http://www.w3.org/1999/xhtml"
    xml:lang="&lt;?php print $language-&gt;language; ?&gt;"
    version="XHTML+RDFa 1.0" dir="&lt;?php print $language-&gt;dir; ?&gt;"
&gt;


where $language is a global object, it prints, in my case:

&lt;html
    xmlns="http://www.w3.org/1999/xhtml"
    xml:lang="en"
    version="XHTML+RDFa 1.0" dir="ltr"
&gt;


Check out the variables available to you in html.tpl.php.

Make sure to clear your cache after you change the template.

EDIT: Full contents of html.tpl.php:

&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"&gt;
&lt;html xmlns="http://www.w3.org/1999/xhtml" xml:lang="&lt;?php print $language-&gt;language; ?&gt;" version="XHTML+RDFa 1.0" dir="&lt;?php print $language-&gt;dir; ?&gt;"
    &lt;?php print $rdf_namespaces; ?&gt;&gt;
    &lt;head profile="&lt;?php print $grddl_profile; ?&gt;"&gt;
        &lt;?php print $head; ?&gt;
        &lt;title&gt;&lt;?php print $head_title; ?&gt;&lt;/title&gt;
        &lt;?php print $styles; ?&gt;
        &lt;?php print $scripts; ?&gt;
    &lt;/head&gt;
    &lt;body class="&lt;?php print $classes; ?&gt;" &lt;?php print $attributes;?&gt;&gt;
        &lt;?php print $page_top; ?&gt;
        &lt;?php print $page; ?&gt;
        &lt;?php print $page_bottom; ?&gt;
    &lt;/body&gt;
&lt;/html&gt;