locales_en.json

{
  "above": "The {{field}} should be above {{argument.0}}.",
  "accepted": "The {{field}} should have been accepted",
  "after": "The {{field}} should be a date after {{argument.0}}",
  "after_offset_of": "The {{field}} should be after {{argument.0}} {{argument.1}} from today’s date",
  "alpha": "The {{field}} should contain letters only",
  "alpha_numeric": "The {{field}} should contain letters and numbers only",
  "array": "The {{field}} should be an ARRAY.",
  "before": "The {{field}} should be a date before {{argument.0}}.",
  "before_offset_of": "The {{field}} should be before {{argument.0}} {{argument.1}} from today’s date",
  "between": "The {{field}} should be between {{argument.0}} and {{argument.1}}.",
  "boolean": "The {{field}} should be true or false.",
  "confirmed": "The {{field}} confirmation does not match.",
  "date": "The {{field}} should be a valid date",
  "date_format": "The {{field}} should be a valid date according to given format {{argument.0}}.",
  "different": "The {{field}} and {{argument.0}} should be different.",
  "email": "The {{field}} should be a valid email address.",
  "ends_with": "The {{field}} should end with given letters ({{argument}}).",
  "exists": "The {{field}} should exists in table {{argument.0}}",
  "equals": "The {{field}} should should equal {{argument.0}}.",
  "in": "The {{field}} should fall within defined values of ({{argument}}).",
  "includes": "The {{field}} should include define letters ({{argument}}).",
  "integer": "The {{field}} should be an INTEGER.",
  "ip": "The {{field}} should be a valid IP address.",
  "ipv4": "The {{field}} should be a valid IPV4 address.",
  "ipv6": "The {{field}} should be a valid IPV6 address.",
  "json": "The {{field}} should be a valid JSON string.",
  "max": "The {{field}} should not be more than {{argument.0}}.",
  "min": "The {{field}} should not be less than {{argument.0}}.",
  "not_equals": "The {{field}} should be different than {{argument.0}}.",
  "not_in": "The {{field}} should not be one of ({{argument}}).",
  "object": "The {{field}} should be a valid OBJECT.",
  "range": "The {{field}} should be between {{argument.0}} and {{argument.1}}.",
  "regex": "The {{field}} format is invalid.",
  "required": "The {{field}} is required.",
  "required_if": "The {{field}} is required when {{argument.0}} exist.",
  "required_when": "The {{field}} is required when value of {{argument.0}} is equal to {{argument.1}}",
  "required_with_all": "The {{field}} is required when all of ({{argument}}) are present.",
  "required_with_any": "The {{field}} is required when any of ({{argument}}) are present.",
  "required_without_all": "The {{field}} is required when none of ({{argument}}) are present.",
  "required_without_any": "The {{field}} is required when any of ({{argument}}) are present.",
  "same": "The {{field}} and {{argument.0}} should match.",
  "starts_with": "The {{field}} should starts with given letters ({{argument}}).",
  "string": "The {{field}} should be a STRING.",
  "under": "The {{field}} should be under {{argument.0}}.",
  "unique": "The {{field}} has already been taken by someone else.",
  "url": "The {{field}} should be a valid URL."
}

"exists": "The {{field}} should exists in table {{argument.0}}",

Mensagem do custom validation que criamos no Behind the code:
"exists": "The {{field}} should exists in table {{argument.0}}",

Message added