Fix #476 Auto language changing function finished.

Fixes issue#476.
If the user does not have the freeseer.conf file, the function will detect the system location for default language.
If there is a config file, the language setting in that file will have higher priority.
For Chinese, If the location is not zh_CN, Traditional Chinese will be used.
If there is no translation for the system location, English will be used.
Just to keep this discussion organized, I'll leave my response here.

Not much code would be duplicated (just `QDir(":/languages").entryList()`?) and it would make everything all the more cleaner. It would probably also clear up your extra Chinese case logic.