Defining language for your pages can be beneficial for SEO in a number of ways;
- helping the search engine to deliver the right content in the right language and region: reducing bounce rate, increasing dwell time,
- allowing the search engine to understand the relationship between pages which might otherwise be considered duplicate,
- allowing the site owner to target different regions that might not be selected by the search engine.
Google provides a comprehensive guide about best practice implementation. We generally advise implementing on-page and in-header to ensure maximum compatibility with all search engines.