If you're using the data annotation attributes and don't want to customize the message for each one (which, it appears by one of your comments that this is the case), I can see two options:
1) Create your own data annotation attribute and set a default message, and change all instances of [Required]
to your new attribute.
2) You can try it in jQuery:
// reset the form's validator
$("form").removeData("validator");
// change the text on each required attribute
$(":input[data-val-required]").attr("data-val-required", "Please enter a value");
// reapply the form's validator
$.validator.unobtrusive.parse(document);
I tried this javascript in Chrome's console and everything seemed to work ok. The reason why you need to do this is because 1) the validation rules are cached by the browser so you need to clear them out (See this answer for more information). And 2) all of the error messages are in the html markup once the page is rendered, so that's where you need to change the validation messages.