Different languages for fronted and backend – $locale not working

Hi,

I want an english admin/dashboard but the frontend (what the public sees) to be in German.

I set my site as German and added this code to my functions.php

function native_admin_lang( $locale ) {
  if ( is_admin() ) {
    return 'en_US';
  }
  return $locale;
}
add_filter( 'locale', 'native_admin_lang' );

But the backend remains in German. I really really really do NOT want to use a plugin.

Any ideas as to why this isn’t working?