Albert Barnes Commentary 2 Chronicles 34:21

"Go ye, inquire of Jehovah for me, and for them that are left in Israel and in Judah, concerning the words of the book that is found; for great is the wrath of Jehovah that is poured out upon us, because our fathers have not kept the word of Jehovah, to do according unto all that is written in this book." — 2 Chronicles 34:21 (ASV)

For them, who are left in Israel and in Judah—Compare the words in 2 Kings 22:13. In both records, the intention is to show that the king regarded the ten tribes as being under his care, no less than the two.