Znalazłem coś takiego, ale nie wiem jak te 2 scrypty powiązać między sobą:
<script LANGUAGE="JavaScript">
var thisString;
function startText() {
thisString = document.niePolskie.box1.value;
thisString = thisString.replace(/ą/g, "a");
thisString = thisString.replace(/Ą/g, "A");
thisString = thisString.replace(/ś/g, "s");
thisString = thisString.replace(/Ś/g, "S");
thisString = thisString.replace(/ę/g, "e");
thisString = thisString.replace(//g, "E");
thisString = thisString.replace(/ć/g, "c");
thisString = thisString.replace(/Ć/g, "C");
thisString = thisString.replace(/ź/g, "z");
thisString = thisString.replace(/Ź/g, "Z");
thisString = thisString.replace(/ż/g, "z");
thisString = thisString.replace(/Ż/g, "Z");
thisString = thisString.replace(/ó/g, "o");
thisString = thisString.replace(/Ó/g, "O");
thisString = thisString.replace(/ń/g, "n");
thisString = thisString.replace(/Ń/g, "N");
thisString = thisString.replace(/ł/g, "l");
thisString = thisString.replace(/Ł/g, "L");
document.niePolskie.box2.value=thisString;
}
</script>
<input type="text" name="box1"> <input type="text" name="box2"> <input type="button" value="Convert" onclick="startText();">