Can't write in other language in my login form

I created a login to my site. That you need to enter username and password:

All is working fine. But the problem is when i write in Hebrew (without clicking the login button) it immediately send an error.

HTML Code:

<section class="container" id="overlay">
  <div class="login" id="overlay">
    <h1>Barometer Login</h1>
    <form method="post" >
      <p>
        <input type="text" name="username" id="username" value="" placeholder="Username" required="required" >
      </p>
      <p>
        <input type="password" name="password" id="password" value="" placeholder="Password" required="required" >
      </p>
      <p class="remember_me"> </p>
      <p class="submit">
        <input class="btn btn-primary" style="background-color:purple;" onClick="SignInFunction(this.value)" name="commit" value="Login">
      </p>
    </form>
  </div>
</section>

JS Code:

function SignInFunction(val){
  var username = document.getElementById("username").value; 
  var password = document.getElementById("password").value; 

 if(username=="Anna" && password=="123"){ 
   document.getElementById("AftetLogin").style.display = "block";
   document.getElementById("overlay").style.display = "none";
 }  
}