Detect and avoid form submit with some Cyrillic character in input string

Question

I would like to build a script that detects and avoid the form submit which its input has some Cyrillic character between any part of the string. Such as: "John Потоцкая Doe" or contains "JohП"

HTML Base:

<form action="#" id="form-validate">
  <input type="text" class="input" id="firstname">
  <button class="submit">teste</button>
</form>  

jQuery script base:

$(document).ready(function() {           
  $('#form-validate').submit(function() {

    if($('input#firstname').val()("")){

    //If "Input" contains a Cyrillic character, for example "John Потоцкая Doe", run the code bellow

     alert('Invalid input character, please use Lating only');
     //Then alert and empty the input
     $('input#firstname').val("")
          return false;
       }                      
  });                       
});

Codepen build base

If someone can help me I would be very grateful! Thanks a lot in advance.