Extern variables on AJAX loaded files

Question

I started a new project and for the first time with language files. I import the language file with this:

if(!isset($_SESSION["lang"])) {  
    $_SESSION["lang"] = "en";  
}  
require_once('./inc/lang/'.$_SESSION["lang"].'.php');

Thats not a problem on a usual index.php:

<div id="login">
  <form method="post">
    <label for="usrName"><?php echo $mainloginPage['userName']; ?></label><br />
    <input type="text" id="usrName" maxlength="25" autofocus />
    <br />
    <br />
    <label for="usrPassword"><?php echo $mainloginPage['password']; ?></label><br />
    <input type="password" id="usrPassword" />
    <p class="center">
      <a href="#" onclick="loadIn('lostPW')"><?php echo $mainloginPage['forgotpw']; ?>?</a>
      <br />
      <br />
      <a href="#" onclick="loadIn('signUp')"><?php echo $mainloginPage['register']; ?></a>
    </p>
    <input type="submit" value="<?php echo $mainloginPage['login']; ?>" />
  </form>
</div>

You see the $mainLoginPage - thats a variable from the language file. When I load another file with AJAX and replace this form part, the variables do not apply and I get errors for not defined variables.

The code that gets loaded in (/inc/templates/lostPW.php):

<form method="post">
   <label for="mail"><?php echo $mainloginPage['mail']; ?>:</label><br />
   <input type="text" id="usrMail" autofocus />
   <p class="center">
      <a href="#" onclick="loadIn('login')"><?php echo $mainloginPage['login']; ?>?</a>
      <br />
      <br />
      <a href="#" onclick="loadIn('signUp')"><?php echo $mainloginPage['register']; ?></a>
   </p>
   <input type="submit" value="<?php echo $main['send']; ?>" />
</form>

The onclick events are just loading a function with a usual xhttp request.

How do I get the variables of this with ajax new loaded file?

loadIn function

function loadIn(file) {
  var xmlhttp = new XMLHttpRequest();
  xmlhttp.onreadystatechange = function() {
      if (this.readyState == 4 && this.status == 200) {
          document.getElementById("login").innerHTML = this.responseText;
      }
  };
  xmlhttp.open("GET", "/inc/templates/" + file + ".php", true);
  xmlhttp.send();
}