When importing data from a file (calling fromFile method, or using the open button of the control) that uses es-ES culture, date cells are rendered as in en-US format, although the value remains in es-ES format.
Here you will find a small Dojo sample, based on the scenario described. Attached you will find an xlsx file, which has a date with Spanish formatting in one of its cells.
After loading that file to the test Spreadsheet, the value in the cell is displayed as it is in the file. The value in the function bar, however, is displayed as dd/MM/yyyy. The reason for that is the fact, that the kendo.culture.es-ES.min.js does not contain a dash-like date format for Spanish culture. Therefore, the Spreadsheet defaults to the "d" format.
Here you will find a short video of running the test.