Change register button label when certain field is selected
First of all you don't need to check if isset is equals 1.

Then I think you must use the form_state or do this with jQuery, like this:

if(document.getElementById('checkbox-is').checked) {
  $("#submit-id").text("Jetzt registrieren");
}


UPDATE: I have edited your code to look like this:

(function ($) {
  $("#edit-field-registrierung-als").change(function() {
    if ($(this).value == "2") {
      $("#edit-submit--2").val("Registrieren");
    }
  }
}) (jQuery);