What are the most popular spreadsheet encodings?

Question

Character encoding

Hello everyone, I need to correctly cast any spreadsheet file to UTF-8 .csv for further work on it. To convert a file, for example, xlsx -> csv, I need to know the source encoding so that I don’t get crocozyabras instead of Cyrillic characters in the output.
Any user with any operating system / program can upload a spreadsheet to the server, so in order to more or less correctly determine the encoding, I need to know the most popular ones that I can theoretically get.
If I'm making a bike, and it's possible to get the result easier, I'll be glad to advice, thanks!