I don't know why the button is in Polish, as far as I understand this is controlled by the browser. As for styling the control, here's a guide you can try:
I haven't tried it myself but it looks pretty comprehensive and well thought out.
and I already know what is going on. This is all because the label on fileUplad button depends on web browser interface language and not on preferred language for dispalying pages. So if you set locale on English but the interface of the browser is Polish the label of the fileUpload button would also be in Polish. And this is not a good message :(
Thanks for the link concerning styling the control.