Replace country code with full name

Where are you seeing this? The country field WooCommerce shows never uses 2 digit abbreviations.

On the last page (final checkout) where the customer details are shown

Are you using the german plugin? Core doesn’t use country code as seen here https://github.com/woothemes/woocommerce/blob/master/includes/class-wc-countries.php#L371 but plugins can change this.