Here's a patch to enable translations

--- class-wc-gateway-ppec-plugin.php.original	2016-08-04 23:45:40.000000000 +0200
+++ class-wc-gateway-ppec-plugin.php	2016-08-04 23:47:40.000000000 +0200
@@ -84,6 +84,7 @@
 	public function maybe_run() {
 		register_activation_hook( $this->file, array( $this, 'activate' ) );

+		add_action( 'plugins_loaded', array( $this, 'load_plugin_textdomain' ) );
 		add_action( 'plugins_loaded', array( $this, 'bootstrap' ) );
 		add_filter( 'plugin_action_links_' . plugin_basename( $this->file ), array( $this, 'plugin_action_links' ) );
 		add_filter( 'allowed_redirect_hosts' , array( $this, 'whitelist_paypal_domains_for_redirect' ) );
@@ -298,4 +299,8 @@
 		return $domains;
 	}

+	public function load_plugin_textdomain() {
+		load_plugin_textdomain( 'woocommerce-gateway-paypal-express-checkout', FALSE, basename( dirname( __FILE__ ) ) . '/languages/' );
+	}
+
 }

https://wordpress.org/plugins/woocommerce-gateway-paypal-express-checkout/