Fix inability to translate to languages which don't have plural forms

Some languages (e.g. Japanese, Chinese, etc.) does not have plural forms. With these languages, `getTranslatedMessage` function returns string value instead of array, even in `translatePlural` function. However, `translatePlural` function can not treat a string value. This issue is critical for us.
IMHO, This `(array) cast` can be into `array($translation)`?, to be clear for fallback.