get_locale() and get_user_locale() are functions to retrieve the locale that is already set. To change it, you want to “filter” the locale value.
For that, you want to use the locale filter hook. The following should do it:
add_filter( 'locale', 'custom_locale' );
function custom_locale( $locale ) {
$locale = ( is_admin() ) ? "en_US" : "de_DE";
return $locale;
}
This should set the admin locale to English, otherwise display German.