Thank you for your answer. I've resolved this issue changing the way I translate the columns header.
Now I'm using the $filter service:
$scope.cols = [
{ field: "CheckoutID", width: 150, visible: false },
{ field: "Extension", headerText: $filter('translate')('ROOM'), width: 180 },
{ field: "CustName", headerText: $filter('translate')('CUSTOMERNAME'), width: 175 },
{ field: "DateTime", headerText: $filter('translate')('DATE'), width: 175, format: dateformat},
{ field: "Calls", headerText: $filter('translate')('CALLS'), width: 180, textAlign: "right", headertextAlign: "right" },
{ field: "Amount", headerText: $filter('translate')('AMOUNT'), width: 110, textAlign: "right", headertextAlign: "right", format: '{0:N2}' }
];