Comment toggling text is not localizable because it's in a .js file.

js/comments.js

@@ -52,13 +52,11 @@ function toggle_all_comments(action) {
 
 function collapse_comment(link, comment) {
     link.innerHTML = "[+]";
-    link.title = "Expand the comment.";
     YAHOO.util.Dom.addClass(comment, 'collapsed');
 }
 
 function expand_comment(link, comment) {
     link.innerHTML = "[-]";
-    link.title = "Collapse the comment";
     YAHOO.util.Dom.removeClass(comment, 'collapsed');
 }
 
@@ -112,11 +110,11 @@ function wrapReplyText(text) {
 /* This way, we are sure that browsers which do not support JS
    * won't display this link  */
 
-function addCollapseLink(count) {
+function addCollapseLink(count, title) {
     document.write(' <a href="#" class="bz_collapse_comment"' +
                    ' id="comment_link_' + count +
                    '" onclick="toggle_comment_display(this, ' +  count +
-                   '); return false;" title="Collapse the comment.">[-]<\/a> ');
+                   '); return false;" title="' + title + '">[-]<\/a> ');
 }
 
 function goto_add_comments( anchor ){

template/en/default/bug/comments.html.tmpl

@@ -146,7 +146,7 @@
               [% END %]
             >reply</a>]
             <script type="text/javascript"><!--
-              addCollapseLink([% count %]); // -->
+              addCollapseLink([% count %], 'Toggle comment display'); // -->
             </script>
           </span>
         [% END %]