Can you explore the island alone or with friends? If not, is there an option to choose a Polish language with a guide?

Yes u can.. over there u get a audio box than u r ur own. U can pause n play the audio when ever u want.

Yes u can..

There are paper guides in several languages as well as the audio tour in multiple languages. There are some areas cut off due to safety concerns, but you can wander around most of the island

There are restricted areas where visitors are not permitted but otherwise you can walk and go anywhere you want at your own pace.

You walk by yourself... You get a audio tour in a language you prefer through the cell house