thanks for your question, Rafael, and thanks for the explanation, e-sushi.
Yes indeed, this is a problem with different character encodings. Unfortunately, however, I have not yet been able to resolve this satisfactorily.
It seems that somewhere between the import process of the CSV file (for which I use an external PHP library) and the saving to the database (which happens in a "serialized" string), the character encoding gets broken, so that special characters (like accents) are not shown correctly.
The only workarounds that I'm currently aware of are to manually save the file again in different character encodings and try with those (this is what e-sushi describes, and you can for example do it with Notepad and "Save as" there).
A different solution (which requires more work) is to manually fix all characters that are broken after the import.
If someone knows a permanent solution (or can even find the necessary steps to fix this in the plugin code), I'd really be happy to hear it!