catalog/language/en-gb/api/coupon.php

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<?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!';