Your PHP enable mbstring extension?
Your spread sheet support utf-8?

Will you check download in file and analysis encoding.
CSV output program(export.php at last)'s encode converting is very simple.
if (function_exists("mb_convert_encoding")) {
    echo mb_convert_encoding($contents, _MD_EXPORT_CHARSET, _CHARSET);
} else {
    echo $contents;
}
If disable "mbstring" else side, that is no conversion (UTF-8 output).
_CHARSET is encoding to use in XOOPS. And _MD_EXPORT_CHARSET destination charset defined in eguide/language/<your?lang>/main.php . Also default "UTF-8".

Please check your self.