Try removing it from the HTML pages, and placing the language identifier info inside the OPF file instead. Though we have had our epub XHTML pages similar as you now have it shown (for past epubs) while our latest epub is set as follows (verified on EpubChecker and live on Kobo).
HTML pages:
Code:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops">
OPF file:
Code:
<dc:language id="language">en</dc:language>
Read this web link for help info at I.S.:
IngramSpark Help