The language can be set from the front panel menu in "Initial Setup". Press Menu, then the up or down arrows to get to "Initial Setup". In there, the very last setting is "Local Language", which can be set to either English or the local language.
Have a look at the Basic User Manual, especially pages 107- 108 (pg 121-122 in the PDF). Unfortunately, in your case all menu entries will be in German, but the manual should be able to guide you. I also searched through the German manual for "deutsch", "sprache" and "language" but they do not appear in the manual. This could mean that the language is not changeable on the German version, but that would be really odd.
As the (English) manual only gives options for English and local language, it would appear that Polish will not be available, and the best you can expect is English.
If all else fails, the entry in "Initial Setup" just before "Local Language" is "Reset", where you can reset the printer to factory defaults. That may set it back to English as well.