Corrected entry: Ghost's sword has runic writing on it which is all well and good except it's written in English. It says "Thunder Sword".
Corrected entry: Horses did not exist in the Americas until the European discovery in the 1400's. The white stallion at the beginning of the film should not have been there.
Correction: The whole plot of the film revolves around European contact with the American continent far earlier than the commonly accepted date in the late 1400's. Given this, it cannot be said that in this fictional history, horses could not have been brought over earlier than the real-world history states.
Correction: As with most movies, period epics are modernized so that English speaking people (or translated language of your choice) can understand/read/hear what is going on without subtitles. Old English, for example, is hardly recognizable compared to modern English, yet movies set in the Middle Ages use modern English. It's a movie making convention, not a mistake, since there are very few people who would be able to decipher Nordic runes today.