You are missing quotes in the alert() call.

alert('<?php echo __("Error-login") ?>'); 

Your line becomes

alert(Error-login);

As you can see, you are missing the quotes:

alert('Error-login');

If somebody uses quotes in the translation, this will also generate an error:

alert('Error's are here');

So you need to escape single quotes before you pass it to Javascript.

try this

<?php echo '<script language="javascript">confirm("Do you want this?")</script>;'; ?>