I became an answer on the jqwidget forums, here the solution:
editor.bind('open', function (event) {
var rows = $('#jqxgridpop').jqxGrid('getboundrows');
for (var i = 0; i < rows.length; i++) {
var value = $('#jqxgridpop').jqxGrid('getcellvalue', i, "languageCode");
var item = editor.jqxDropDownList('getItemByValue', value);
editor.jqxDropDownList('disableItem', item);
};
});