Modifying Wordpress plugin

i have a trouble with plugin, script reads the first characters (A,B,C.. from letters) but if it is Č (or different language special characters) wrotes "Sorry, no posts were found!" here is a screen: http://ift.tt/1O2EIr3 please advise where is the problem,thank you

                    <?php query_posts('post_parent=83&post_type=page&post_status=publish&orderby=title&order=ASC'); ?>
                    <?php if ( have_posts() ) {
                        $in_this_row = 0;
                        while ( have_posts() ) {
                           the_post();
                           $first_letter = mb_strtoupper(substr(apply_filters('the_title',$post->post_title),0,1));
                           if ($first_letter != $curr_letter) {
                              if (++$post_count > 1) {
                                 end_prev_letter();
                              }
                              start_new_letter($first_letter);
                              $curr_letter = $first_letter;
                           }
                           if (++$in_this_row > $posts_per_row) {
                              end_prev_row();
                              start_new_row();
                              ++$in_this_row;  // Account for this first post
                           } ?>

                           <div class="title-cell"><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></div>
                        <?php }
                        end_prev_letter();
                        ?>
                        <div class="navigation">
                           <div class="alignleft"><?php next_posts_link('&laquo; Higher Letters') ?></div>
                           <div class="alignright"><?php previous_posts_link('Lower Letters &raquo;') ?></div>
                        </div>
                     <?php } else {
                        echo "<h2>Sorry, no posts were found!</h2>";
                     }
                    ?>


                     <?php
                        function end_prev_letter() {
                           end_prev_row();
                           echo "</div><!-- End of letter-group -->\n";
                           // echo "<div class='clear'></div>\n";
                        }
                        function start_new_letter($letter) {
                           echo "<div class='letter-group'>\n";
                           echo "\t<div class='letter-cell'>$letter</div>\n";
                           start_new_row($letter);
                        }
                        function end_prev_row() {
                           echo "\t</div><!-- End row-cells -->\n";
                        }
                        function start_new_row() {
                           global $in_this_row;
                           $in_this_row = 0;
                           echo "\t<div class='row-cells'>\n";
                        }
                    ?>

via Chebli Mohamed