UTF-8 can encode any Unicode character, preventing the need to set a encoding type for every language that is going to be used in the site and allowing output in multiple languages at the same time. I would recommend to use this type of encoding in case you are setting up a multilingual e-shop on a MySQL5 / PHP5 platform.

How to adjust osCommerce to use UTF-8

1. For every language you use edit the main language file (for example: catalog/includes/languages/greek.php and catalog/admin/includes/languages/greek.php) below:
     @setlocale(LC_TIME, 'xx_XX.UTF8'); //xx_XX stands for your language
   Add
     mb_internal_encoding("UTF-8");
2. In catalog(/admin)/includes/functions/database.php add mysql_query("SET NAMES 'UTF8'", $$link); after the lines
   if (USE_PCONNECT == 'true') {
         $$link = mysql_pconnect($server, $username, $password);
       } else {
         $$link = mysql_connect($server, $username, $password);
       }
3. Check that the (text) table fields in your database are set to the utf8_general_ci collation.
4. Make sure the language packs you are going to use are encoded in and set to UTF-8.
5. In catalog/includes/functions/general.php replace the complete function tep_parse_search_string with:
       function tep_parse_search_string($search_str = '', &$objects) {
       $search_str = trim(mb_strtolower($search_str));
   
   // Break up $search_str on whitespace; quoted string will be reconstructed later
       $pieces = mb_split('[[:space:]]+', $search_str);
       $objects = array();
       $tmpstring = '';
       $flag = '';
   
       for ($k=0; $k<count($pieces); $k++) {
         while (mb_substr($pieces[$k], 0, 1) == '(') {
           $objects[] = '(';
           if (mb_strlen($pieces[$k]) > 1) {
             $pieces[$k] = mb_substr($pieces[$k], 1);
           } else {
             $pieces[$k] = '';
           }
         }
   
         $post_objects = array();
   
         while (mb_substr($pieces[$k], -1) == ')')  {
           $post_objects[] = ')';
           if (mb_strlen($pieces[$k]) > 1) {
             $pieces[$k] = mb_substr($pieces[$k], 0, -1);
           } else {
             $pieces[$k] = '';
           }
         }
   
   // Check individual words
   
         if ( (mb_substr($pieces[$k], -1) != '"') && (mb_substr($pieces[$k], 0, 1) != '"') ) {
           $objects[] = trim($pieces[$k]);
   
           for ($j=0; $j<count($post_objects); $j++) {
             $objects[] = $post_objects[$j];
           }
         } else {
   /* This means that the $piece is either the beginning or the end of a string.
      So, we'll slurp up the $pieces and stick them together until we get to the
      end of the string or run out of pieces.
   */
   
   // Add this word to the $tmpstring, starting the $tmpstring
           $tmpstring = trim(mb_ereg_replace('"', ' ', $pieces[$k]));
   
   // Check for one possible exception to the rule. That there is a single quoted word.
           if (mb_substr($pieces[$k], -1 ) == '"') {
   // Turn the flag off for future iterations
             $flag = 'off';
   
             $objects[] = trim($pieces[$k]);
   
             for ($j=0; $j<count($post_objects); $j++) {
               $objects[] = $post_objects[$j];
             }
   
             unset($tmpstring);
   
   // Stop looking for the end of the string and move onto the next word.
             continue;
           }
   
   // Otherwise, turn on the flag to indicate no quotes have been found attached to this word in the string.
           $flag = 'on';
   
   // Move on to the next word
           $k++;
   
   // Keep reading until the end of the string as long as the $flag is on
   
           while ( ($flag == 'on') && ($k < count($pieces)) ) {
             while (mb_substr($pieces[$k], -1) == ')') {
               $post_objects[] = ')';
               if (mb_strlen($pieces[$k]) > 1) {
                 $pieces[$k] = mb_substr($pieces[$k], 0, -1);
               } else {
                 $pieces[$k] = '';
               }
             }
   
   // If the word doesn't end in double quotes, append it to the $tmpstring.
             if (mb_substr($pieces[$k], -1) != '"') {
   // Tack this word onto the current string entity
               $tmpstring .= ' ' . $pieces[$k];
   
   // Move on to the next word
               $k++;
               continue;
             } else {
   /* If the $piece ends in double quotes, strip the double quotes, tack the
      $piece onto the tail of the string, push the $tmpstring onto the $haves,
      kill the $tmpstring, turn the $flag "off", and return.
   */
               $tmpstring .= ' ' . trim(mb_ereg_replace('"', ' ', $pieces[$k]));
   
   // Push the $tmpstring onto the array of stuff to search for
               $objects[] = trim($tmpstring);
   
               for ($j=0; $j<count($post_objects); $j++) {
                 $objects[] = $post_objects[$j];
               }
   
               unset($tmpstring);
   
   // Turn off the flag to exit the loop
               $flag = 'off';
             }
           }
         }
       }
   
   // add default logical operators if needed
       $temp = array();
       for($i=0; $i<(count($objects)-1); $i++) {
         $temp[] = $objects[$i];
         if ( ($objects[$i] != 'and') &&
              ($objects[$i] != 'or') &&
              ($objects[$i] != '(') &&
              ($objects[$i+1] != 'and') &&
              ($objects[$i+1] != 'or') &&
              ($objects[$i+1] != ')') ) {
           $temp[] = ADVANCED_SEARCH_DEFAULT_OPERATOR;
         }
       }
       $temp[] = $objects[$i];
       $objects = $temp;
   
       $keyword_count = 0;
       $operator_count = 0;
       $balance = 0;
       for($i=0; $i<count($objects); $i++) {
         if ($objects[$i] == '(') $balance --;
         if ($objects[$i] == ')') $balance ++;
         if ( ($objects[$i] == 'and') || ($objects[$i] == 'or') ) {
           $operator_count ++;
         } elseif ( ($objects[$i]) && ($objects[$i] != '(') && ($objects[$i] != ')') ) {
           $keyword_count ++;
         }
       }
   
       if ( ($operator_count < $keyword_count) && ($balance == 0) ) {
         return true;
       } else {
         return false;
       }
     }
6. Correct the function tep_break_string (catalog(admin)/includes/functions/general.php):
     function tep_break_string($string, $len, $break_char = '-') {
       $l = 0;
       $output = '';
       for ($i=0, $n=strlen($string); $i<$n; $i++) {
         $char = substr($string, $i, 1);
         if ($char != ' ') {
           $l++;
         } else {
           $l = 0;
         }
         if ($l > $len) {
           $l = 1;
           $output .= $break_char;
         }
         $output .= $char;
       }
   
       return $output;
     }
   to:
     function tep_break_string($string, $len, $break_char = '-') {
       $l = 0;
       $output = '';
       for ($i=0, $n=mb_strlen($string); $i<$n; $i++) {
         $char = mb_substr($string, $i, 1);
         if ($char != ' ') {
           $l++;
         } else {
           $l = 0;
         }
         if ($l > $len) {
           $l = 1;
           $output .= $break_char;
         }
         $output .= $char;
       }
   
       return $output;
     }
7. In catalog/(admin/)/includes/classes/email.php change:
        if ((strstr($to_name, "\n") != false) || (strstr($to_name, "\r") != false)) {
           return false;
         }
   
         if ((strstr($to_addr, "\n") != false) || (strstr($to_addr, "\r") != false)) {
           return false;
         }
   
         if ((strstr($subject, "\n") != false) || (strstr($subject, "\r") != false)) {
           return false;
         }
   
         if ((strstr($from_name, "\n") != false) || (strstr($from_name, "\r") != false)) {
           return false;
         }
   
         if ((strstr($from_addr, "\n") != false) || (strstr($from_addr, "\r") != false)) {
           return false;
         }
   
         $to = (($to_name != '') ? '"' . $to_name . '" ' : $to_addr);
         $from = (($from_name != '') ? '"' . $from_name . '" ' : $from_addr);
   to:
         if ((mb_strstr($to_name, "\n") != false) || (mb_strstr($to_name, "\r") != false)) {
           return false;
         }
   
         if ((mb_strstr($to_addr, "\n") != false) || (mb_strstr($to_addr, "\r") != false)) {
           return false;
         }
   
         if ((mb_strstr($subject, "\n") != false) || (mb_strstr($subject, "\r") != false)) {
           return false;
         }
   
         if ((mb_strstr($from_name, "\n") != false) || (mb_strstr($from_name, "\r") != false)) {
           return false;
         }
   
         if ((mb_strstr($from_addr, "\n") != false) || (mb_strstr($from_addr, "\r") != false)) {
           return false;
         }
   
         $tommod_encoding = strtolower(constant('CHARSET'));
         if($tommod_encoding=="utf-8"){
         mb_internal_encoding("utf-8");
         $subject = mb_encode_mimeheader($subject, "utf-8", "q");
         $from_name = '';
         $to_name = '';
   
   
         if(mb_ereg("<", $from_addr))
         {
         $froma = mb_split("<", $from_addr);
         $from_addr = str_replace(">", "", $froma[1]);
         }
        
         if(mb_ereg("<", $to_addr))
         {
         $toa = mb_split("<", $to_addr);
         $to_addr = str_replace(">", "", $toa[1]);
         }     
         }
   
         $to = (($to_name != '') ? '"' . $to_name . '" ' : $to_addr);
         $from = (($from_name != '') ? '"' . $from_name . '" ' : $from_addr);