Rocket Validator looks for accesibility violations on your pages using the axe-core accessibility engine, hosted on our own servers and integrated into our web crawler. You don't need to buy an additional license, everything's already included in your Pro account at Rocket Validator.
A11Y check: HTML elements with lang and xml:lang must have the same base language
Accessibility rule: The HTML document must contain a valid lang attribute or must correspond to a valid lang code for multilingual screen reader users who may prefer a language other than the default. The xml:lang attribute value, if included on the html element, must duplicate the value of the lang exactly.