Problem:

Installing Simple Tags (1.5.7) on WordPress with Gengo (2.5.3) results in a classic localization problem:

Fatal error: Call to a member function on a non-object in [...]/wp-includes/classes.php on line 38

Solution:

Open [...]/wp-content/plugins/simple-tags/2.5/simple-tags.client.php and find in SimpleTags() function:

// Localization
$locale = get_locale();
if ( !empty( $locale ) ) {
   $mofile = str_replace('/2.5', '', $this->info['install_dir'])
      .'/languages/simpletags-'.$locale.'.mo';
   load_textdomain('simpletags', $mofile);
}

Remove this part and place it in a separate function (just before add_action(‘plugins_loaded’, ‘st_init’);) like this:

function locale_init() {
   global $simple_tags;

   // Localization
   $locale = get_locale();
   if ( !empty( $locale ) ) {
      $mofile = str_replace('/2.5', '', $simple_tags->info['install_dir']).
         '/languages/simpletags-'.$locale.'.mo';
      load_textdomain('simpletags', $mofile);
   }
}

After add_action(‘plugins_loaded’, ‘st_init’); add this line:

add_action('init', 'locale_init');

This way the localization part gets postponed till Gengo establishes all the needed stuff. Use at your own risk – not thoroughly tested yet.