What Does "Missing HTML lang value" Mean?
The URLs are missing an HTML language tag value.
The HTML language declares the default language for all content on the page. To be implemented correctly, the lang attribute should be applied to the opening <html> tag at the top of every page (just under the doctype).
It will typically look like this:
Why It's Important
If the language attribute is missing on the page, it's difficult for search engines and browsers to know what language the content of the page is. When this happens, it's possible the page may not be displayed correctly or search engines could have difficulties processing the page which could lead to poor rankings or ranking in the wrong language's search results.
It can also cause accessibility issues to visitors to your site, especially those using screen readers.
How To Fix
Add the lang attribute in the opening <html> tag at the top of the page just under the doctype. The value should be the appropriate language tag that represents the content on this page. For the vast majority of pages on the web, this is simply the ISO-639 two-letter language code, or ISO-639 two-letter code with ISO-3166 two-letter country code. Simply visit a code lookup table, and find the code that matches your content.
For more specific targeting there can be quite complicated rules for region subtags, variant subtags, and other special cases. If you're not sure which code to use, we recommend following instructions available from the W3.