SEOptimer's check identifies the presence of the Lang attribute in your site's HTML. Having this attribute is highly recommended to prevent language confusion or misinterprataion by browsers or Search Engines.
The lang attribute is used to describe the intended language of the current page to user's browsers and Search Engines. Search Engines may use the lang attribute to return language specific search results to a searcher, and in the browser, lang attribute can signal the need to switch to a different language if it is different to the user's own preferred language.
We recommend adding the lang attribute to the HTML tag of every page to avoid any chance of misinterpretation of language. This may need to be manually added to the site's HTML code, or may be controlled by your CMS.