Complex sentence: The Indo-Iranian Saka people, speaking a language belonging to the Iranian branch of the Indo-European languages, were known as the Sai 塞 (Sāi, also called Sairen 塞人 or Saizhong 塞種) in ancient Chinese records.
Split into simpler sentences: The Indo-Iranian Saka people, speaking a language belonging to the Iranian branch of the Indo-European languages. They were known as the Sai 塞 (Sāi, also called Sairen 塞人 or Saizhong 塞種) in ancient Chinese records.