Instead of opening CSV file in Excel, create a blank document and import CVS into it. On the ribbon click Data-> From Text/CSV
In the data import dialog there will be an option to choose encoding, select "Unicode UTF-8"
Also, in "Browser invoke download" action set MIME type to "text/csv"
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By the way, you can actually create files without user interaction. Add NW.js object to your project and use "NWjs Write File". For example, you can ask user to pick a folder ("NWjs Show Folder dialog") and then export multiple files into it with one click.