Make plexconnect.xml, search.xml, versionError.xml translatable and update translations.
Make more string able to be translate.
Get language setting from HTTP header.
Remove local language setting.
If preferred language is not supported by PlexConnect, use English by default.
Refine some string id.
Refine XML. AppEntry.xml and SearchForm.xml are served by XMLConvertor now, we should use URL() instead of hard code.