Yes, it needs to be in Polish.
I remember a friend (well, 'acquaintance' would be more accurate) having his tax registration form rejected because he'd used non-Polish characters (Kevin Q, remember him?).
The foreigners office in Warsaw have English-language forms but insist that those are filled in in Polish. I once had an application rejected because I'd written "Brown" the space for the response to "Eyes", and then had it rejected again for writing "Mam".