Update lang.php

Modified clean_tables() to really clean up unused language entries in all content language tables and setting table.
Update lang.php

clean_tables(): Moved deleting code into if block.
Update lang_model.php

Added function list_lang_tables() which returns all content language table names.
Update settings_model.php

Added function clean_lang_settings() which cleans up unused language entries in setting table.
Merge pull request #161 from kochinc/master

Thanks for this update !