Corrected entry: Near the end, Karin reads the Bible aloud. She speaks Swedish. However, the movie takes place during the crusades and the Bible wasn't translated to Swedish until 1500.
Corrected entry: The knight and his squire are returning to Sweden from the Crusades in the midst of the Black Plague. It is revealed that they set out for the Holy Land ten years before the film is set, which would make their departure between 1338 and 1342, by the accepted dates of the bubonic plague. However, the ninth, and last, Crusade ended around 1290, so the dates don't add up.
Correction: Debatable. The EIGHTH Crusade did in fact end in 1291 with the fall of Acre and thus the last Christian holding in the Holy Land, and that is considered the last "true" Crusade, but military expeditions to that area remained common until the fall of Constantinople in 1453 and are considered by some to be minor crusades, or at least similar campaigns.
Correction: This can be explained as a simple film convention. It's the same as English language films set in times before English existed featuring English-speaking characters; Ingmar Bergman just used Swedish so his home audience could understand it.