[ticket/11361] Make sure that array passed to strtr() has the proper format.

The array $date_cache[$format]['lang'] passed to strtr() contains a sub-array
which results in an E_NOTICE being thrown for 'Array to string conversion' on
PHP 5.4.
Ensure that the array passed to strtr() is one-dimensional by filtering out
non-string values.

PHPBB3-11361
Merge remote-tracking branch 'github-bantu/ticket/11361' into develop-olympus

* github-bantu/ticket/11361:
  [ticket/11361] Make sure that array passed to strtr() has the proper format.