romm,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<form name="test" method="post">
<p>Текст:<Br>
<textarea name="comment" cols="40" rows="3"></textarea></p>
</form>
<form name="test2" method="post">
<p>Перевод:<Br>
<textarea name="comment" cols="40" rows="3"></textarea></p>
</form>
<script>
window.addEventListener('DOMContentLoaded', function() {
var comment = document.querySelectorAll('[name="comment"]');
comment[0].addEventListener('input', function() {
comment[1].value = this.value
});
});
</script>
</body>
</html>