Laravel - Validation - Quick Customizing The Error Messages
Laravel's built-in validation rules each has an error message that is located in your application's
resources/lang/en/validation.php file. Within this file, you will find a translation entry for each validation rule. You are free to change or modify these messages based on the needs of your application.
In addition, you may copy this file to another translation language directory to translate the messages for your application's language. To learn more about Laravel localization, check out the complete localization documentation.