First, make sure you're working with a CSV file and not an Excel Workbook, then convert all the cells without numbers to plain text formatting by right-clicking, selecting "Format Cell," and choosing "Text." You can also create custom cells for elements such as digit-based codes, but plain text should work just fine. Save your file and update the linked CSV file in InDesign. When you preview your list in InDesign, your imported data with special characters will display properly.