Finaly I figured out where the site name of the titleline was inserted into the site. Just as with the Slogan, it's in the header.php file.
The title line is divided into two parts:
The first part is the site name, set using the admin->preferances->general settings form.
The second part is a slogan which changes for each module you enter - and for the startpage is is set in same place as the site name.
In a previous posting in this thread, I explained the four placed I changed to change the slogan. I at the time that two of the four changes would cover the site name and the other two would cover the slogan - not so!
In order to change the site name, I replaced the following line
$xoopsTpl->assign(array('xoops_theme' => $xoopsConfig['theme_set'], 'xoops_imageurl' => XOOPS_URL.'/themes/'.$xoopsConfig['theme_set'].'/', 'xoops_themecss'=> xoops_getcss($xoopsConfig['theme_set']), 'xoops_requesturi' => htmlspecialchars($GLOBALS['xoopsRequestUri'], ENT_QUOTES), 'xoops_sitename' => $xoopsConfig['sitename'], 'xoops_slogan' => $xoopsConfig['slogan']));
...with the following lines
$myts =& MyTextSanitizer::getInstance();
$xoopsTpl->assign(array('xoops_theme' => $xoopsConfig['theme_set'], 'xoops_imageurl' => XOOPS_URL.'/themes/'.$xoopsConfig['theme_set'].'/', 'xoops_themecss'=> xoops_getcss($xoopsConfig['theme_set']), 'xoops_requesturi' => htmlspecialchars($GLOBALS['xoopsRequestUri'], ENT_QUOTES), 'xoops_sitename' => $myts->makeTboxData4Show($xoopsConfig['sitename']), 'xoops_slogan' => $xoopsConfig['slogan']));
and voila, the browsertitle is multilangual.
(Phew - finally I found it