When a user changes locale (or when we automatically detect and change it for them), we will change the URL of the page they are in. We can either change the path of the URL to prefix it with the locale code, or we can add a query parameter to the URL. We also use this mode to detect the locale for the current page when a user directly loads a page. When NULL, locale detection from URL will be disabled (even then, if the user has selected a locale manually, and followUser is enabled, we will still automatically translate the page in user's locale.