Google saves the preferred language in a cookie setting, which you can manually reset via hamburger menu -> Search Settings -> Languages or with the &hl= CGI parameter. Most of the time it's autodetected via the queries you type though - the only time it's likely to be wrong is if you type a query that's in an ambiguous language (eg. a proper noun), have no cookies (eg. incognito, or first time searching with the browser), and are not signed in. Then it defaults to the geographic location.