html lang=”” instead of lang=”en-us” – why?

You can apply the language_attributes filter to language_attributes() function (source).

Basically you can do this by adding something like this to your functions.php:

add_filter('language_attributes', 'custom_lang_attr');
function custom_lang_attr() {
  return 'lang="en-US"';
}

Note: Keep in mind, that you’re overwriting the language parameter; the original string (see @param) is a space-separated list of language attributes.