Page language not set
WCAG 3.1.1
The <html> tag should say what language the page is in, for example <html lang="en">. Screen readers use it to pick the right voice and pronunciation. Without it, an English page can be read with a French accent, or the reverse.
On hosted platforms this comes from your site language setting, so it's usually a one-minute fix. On custom sites, add the lang attribute to the <html> tag in your layout template.