In the file "index.template.php" the search button is "hardcoded" in line 407.
It is
if ($context['allow_search'])
echo '<li><a' , $current_action=='search' ? ' class="chosen"' : '' , ' href="', $scripturl, '?action=search"><b>Search</b></a></li>';
Instead of
if ($context['allow_search'])
echo '<li><a' , $current_action=='search' ? ' class="chosen"' : '' , ' href="', $scripturl, '?action=search"><b>' , $txt[182] , '</b></a></li>';