Change frontend and backend language

How t change locale for Front-end and Back-end for multisite.
for eg. set locale to Germany for front end and English for Dashboard

For admin locale
In general Setting set default language to : English (en_US) as for English

In function.php (I am using child theme)

add_filter('locale', 'set_frontend_locale');
function set_frontend_locale($locale) {
  return 'de_DE';
}

Hope this will work for you.