add zh-CN, update ar & pl-PL and fix l10n bug

1. add zh-CN language
2. update at & pl-PL languages
3. fix an l10n bug. If users are using languages other than en.ini, there might be chances that current language doesn't have this related key because the whole process would take time in getLocalization.com. In this way, we will fallback to en.ini directly to show en strings instead of make the application broken.