Mixed types invoke notices.

While I add the translation I receive this notices.

It is used in the code:

 return (
      user_access('translate configuration') &&
        $group->hasSchema() &&
        $group->hasTranslatable() &&
        !$group_language->locked &&
        (empty($language) || (!$language->locked && $language->langcode != $group_language->langcode))
    );

$group_language - mixed(object|FALSE)

The first way is to complicate conditions. I think the main problem is that we define The English as the built-in language. Built-in language does not has a key - en. There two extra built-in languages. I do not know why exactly.

The problem in here:
modules/config_translation/lib/Drupal/config_translation/Controller/ConfigTranslationController.php

 if (!isset($languages[$original_langcode])) {
      $language_name = language_name($original_langcode);
      if ($original_langcode == 'en') {
        $language_name = t('Built-in English');
      }
      // Create a dummy language object for this listing only.
      $languages[$original_langcode] = new Language(array('langcode' => $original_langcode, 'name' => $language_name));
    }