const incident_comments = document.getElementById('incident.comments');
const sc_req_item_comments = document.getElementById('sys_original.sc_req_item.comments');

switch (document.getElementById('sys_readonly.sc_req_item.u_language').value) {
  case 'en':
    alert('EN');
    if (incident_comments) incident_comments.value = 'something in english';
    if (sc_req_item_comments) sc_req_item_comments.value = 'something in english';
    break;
  case 'es':
    alert('ES');
    if (incident_comments) incident_comments.value = 'something in spanish';
    if (sc_req_item_comments) sc_req_item_comments.value = 'something in spanish';
    break;
}