Based on the trailer it looks to have been filmed in English with Polish subtitles. But maybe it was dubbed. If so it was dubbed well.
Except for F. Murray Abraham all of the actors and the entire production seem to be Polish. Which is wonderful. A Hollywood production would probably portray the Janissary horde as the good guys. This film and a reminder of the history of this endless (1,400 years) war is very timely.