craft()->on('entries.onBeforeSaveEntry', function(Event $event) {
  $entry = $event->params['entry'];
  $turkishCharacters = ['ı', 'ü', 'ö', 'ş', 'ç', 'ğ'];
  $englishCharacters = ['i', 'u', 'o', 's', 'c', 'g'];

  $entrySlug = $entry->slug;
  $entrySlug = str_replace($turkishCharacters, $englishCharacters, $entrySlug);
  $entry->slug = $entrySlug;
});