i18n: Implement system language management

include/ajax.i18n.php

include/class.config.php

include/class.i18n.php

include/class.translation.php

include/client/header.inc.php

include/client/profile.inc.php

include/i18n/en_US/help/tips/settings.system.yaml

include/staff/profile.inc.php

include/staff/settings-system.inc.php