Most screen readers can read text in several languages and allow users to choose their default language during setup. When your HTML doesn't have a specified
lang, this default option kicks in, and if there's a mismatch the user might hear the wrong accent or improper pronunciation, making the text incomprehensible.
lang attribute to your HTML is an easy fix that helps render the text more accurately, load the correct pronunciation rules, and show video captions correctly. It's also part of the minimum requirements under the Web Content Accessibility Guidelines (WCAG).