need to restrict fields to accept text in only one language Arabic only or English only

You can use the following code as the custom validation script of your field to reject any values that contain English letters:

if (/[a-z]/gi.test(event.value)) {
    app.alert("Error! This field may not contain any English letters.");
    event.rc = false;
}

And this one to reject any values that contain Arabic letters:

if (/[\u0600-\u06FF]/g.test(event.value)) {
    app.alert("Error! This field may not contain any Arabic letters.");
    event.rc = false;
}