Issue #3104071 by jungle, knyshuk.vova: DrupalDateTime::$formatTranslationCache should be an array

  use StringTranslationTrait;

  /**

   * Format string translation cache.

   * Formatted strings translation cache.

   *

   * @var string

   * Translation cache represents an instance storage for formatted date

   * strings. It contains a multidimensional array where:

   * - first level keys - are drupal language codes;

   * - second level keys - are each symbols of given format string (like 'F');

   * - third level keys - are original matched strings related to the symbol;

   * - values - are translated or not-translated original strings (depends on

   *   if a particular symbol represents translatable value according to PHP's

   *   date() format character).

   *

   * For example:

   * @code

   *   [

   *     'en' => [

   *       'F' => [

   *         'November' => t('November'),

   *         'December' => t('December'),

   *       ],

   *       'd' => [

   *         '10' => '10',

   *         '31' => '31',

   *       ],

   *     ],

   *   ]

   * @endcode

   *

   * @var array

   */

  protected $formatTranslationCache;