Supplemental data lists CN with Sunday as the first day of week. According to Chinese National Standard GB/T 7408-2005, which adopts ISO 8601-2000, section 5.2.1 (page 8), the first day of week should be Monday.
the GB/T 7408-2005 standard link on github:
Part of current supplemental data is show below:
<firstDay day="sun" territories="
AG AS AU
BD BR BS BT BW BZ
CA CN CO