Define Language
Is English your first language? Even if English is your first language, and you recognize it on a website that clearly looks like English, but is not defined as the main language, you'll find that assistive technology like screen readers can't always guess that language correctly when reading the text. The same set of letters that create a word can mean something different in a variety of languages and be pronounced differently, confusing screen reader users.
Therefore, all documents and websites should have the default, or main, language defined explicitly. If a portion of the webpage is in another language, like a quote, then the language for that quote needs to be defined explicitly.