Adicione isso a um arquivo JS e inclua após o plugin jQuery Validate
jQuery.extend(jQuery.validator.messages, {
required: "This field is required.",
remote: "Please fix this field.",
email: "Please enter a valid email address.",
url: "Please enter a valid URL.",
date: "Please enter a valid date.",
dateISO: "Please enter a valid date (ISO).",
number: "Please enter a valid number.",
digits: "Please enter only digits.",
creditcard: "Please enter a valid credit card number.",
equalTo: "Please enter the same value again.",
accept: "Please enter a value with a valid extension.",
maxlength: jQuery.validator.format("Please enter no more than {0} characters."),
minlength: jQuery.validator.format("Please enter at least {0} characters."),
rangelength: jQuery.validator.format("Please enter a value between {0} and {1} characters long."),
range: jQuery.validator.format("Please enter a value between {0} and {1}."),
max: jQuery.validator.format("Please enter a value less than or equal to {0}."),
min: jQuery.validator.format("Please enter a value greater than or equal to {0}.")
});