Siteimprove has a check for this requirement. It looks to ensure that the lang attribute is applied to the <html> tag at the start of the document. For most Cornell based sites, the lang attribute here should read "en" or "en-US" for American English.
If you do run across a site that is predominantly in a different language other than English, then the lang attribute needs to have the value which correlates to the predominant language of the page.
When testing pages in an alternate language than your default (probably English if you are reading this), you need to ensure that your screen reader is set up to have voices enabled for the language you are testing. On Mac, you can set this up in VoiceOver by going to your VoiceOver Utility, select "Speech" and click the "Voices" tab. From there, press the plus (+) button to assign voices to different languages.
