Added default language pack (en-gb)

catalog/api/coupon.php

@@ -0,0 +1,7 @@
+<?php
+// Text
+$_['text_success']     = 'Success: Your coupon discount has been applied!';
+
+// Error
+$_['error_permission'] = 'Warning: You do not have permission to access the API!';
+$_['error_coupon']     = 'Warning: Coupon is either invalid, expired or reached it\'s usage limit!';