How to send language string in email message body

You have a typo in your code, the function is __() with two underscores. You will also need to configure the textdomain ‘text-domain’ and have a valid translation for the end users’ browser settings for it to not default to english.

// use two underscores in the method
$message  = __("Hello Admin,",'text-domain');

// assuming this is a plugin and your language files are in a /languages subdirectory
function load_textdomain() {
    load_plugin_textdomain( 'text-domain', false, dirname( plugin_basename( __FILE__ ) ) . '/languages/' );
}
add_action( 'plugins_loaded', 'load_textdomain' );