Set the page language in WordPress

1. Go to Settings > General and choose the right Site Language. Save.
2. WordPress prints the language on the <html> tag through the theme. Block themes and well-made classic themes do this automatically.
3. If the scan still reports it missing, your theme's header.php probably has a bare <html> tag. A developer should change it to <html <?php language_attributes(); ?>>, or switch to a maintained theme.
4. Multilingual plugins (WPML, Polylang) set the language per page; check their settings if only some pages fail.

Check it: publish your change, then run a new scan from your dashboard. The issue should drop off the list.