The 13th Warrior (1999)
Though it is not stated why a Northman could understand both Latin and Greek as there were no official connection between Scandinavia and Byzantine in the 10th century, the logical assumption would be that Herger learned the languages due to him taking part in the viking raids/travels into modern day Russia (where the first scenes are filmed) and from there south into what was then the Byzantine Empire. The dominant language of the Byzantium realm was Greek with Latin as a second language. The vikings made at least one attempt at conquering the capital of the Byzantium Empire: Constantinople.
1 - Buliwyf, Son of Hygiliak. In the movie, perishes from poison delivered by the Wendol Mother. In the book, Buliwyf is dying from the poison but his body is hacked up in the final battle and he dies a bloody death.
2 - Helfdane the Fat. In the movie, he dies from a chest wound in the Wendol lair. In the book, he is a warrior killed in the second encounter with the fire worm.
3 - Ragnar the Dour. He is decapitated in the first encounter in the Great Hall. Strangely, Ragnar is one of four survivors in the book.
4 - Edgtho the Silent. The masterful scout in the movie. In the book, he is called Ecthgow, and is Buliwyf's lieutenant. He is decapitated in the final battle.
5 - Rethel the Archer. In the movie, he is killed when his position on the rooftop is overrun by Wendol during the second battle (the first fight after the sighting of the Fireworm). In the book, he is a younger warrior who is mortally wounded (stomach / intestinal wound) in the second encounter.
6 - Haltaf the Boy. Rethel's son in the movie. He appears briefly in the film; it's Haltaf who snipes the Wendol sentry at the lair. Haltaf is also the one who waves to Ahmed during the farewell at the beach. In the book, he is the second of four survivors, and Crichton makes no mention if Haltaf and Rethel are related.
7 - Roneth the Unspoken. In the movie, he is killed in the second encounter once the barricade is torn down (the camera moves up to his astonished look, and a Wendol cavalryman charges through him). In the book, he is an Earl who had his chest torn open in the first encounter at the Great Hall.
8 - Weath the Musician. The redhead who looks like Herger in the movie. In the book, he is a warrior who was speared through the chest in the final encounter.
9 - Hyglak the Quarrelsome. He is killed in the first encounter in the Great Hall (they find his headless corpse in the aftermath). In the book, he is a warrior who was decapitated in the first encounter. His name was changed in the film as well, because his name, 'Edgtho', likely because it appeared too closely to Ecthgow (the lieutenant).
10 - Halga the Wise. In the movie, he is killed by cavalry lances / spears being run through his body in the second encounter. In the book, he is an Earl who had his spine broken in the first encounter.
11 - Herger the Joyous. The talkative bloke. He is mostly portrayed as he is in the book, and he is the third of four survivors.
12 - Skeld the Superstitious. In the movie, he is killed by arrows or thrown spears when he is attempting to dislodge burning arrows on a fortification during the second encounter. In the book, he is an Earl killed in the second encounter with the fire worm.
13 - Ahmed Ibn Fahdlan. 'Eban', the 13th Warrior. Ahmed is more or less portrayed as he was described in the book. He is the last of the four survivors of this adventure.
Additionally, there is an extra scene at the end of the book where Herger and Ahmed battle Hrothgar's "evil" son, Wiglaf and one of his henchmen in a final duel.
The roles of Melchisidek (Omar Shariff) and Olga (Marie Bonnevie) were taken from an old man who "spoke Greek" and warned Ahmed of Prince Wiglaf's ambition. Ahmed has no romantic interest in the book (any woman he slept with were unnamed) and Queen Weilew (Diane Venora) did not play as large a role (power behind the throne) in the book as she does in the movie.
The one-eyed old man (Richard Ooms) who describes the fire worm is the only remnant of the Greek speaking old man from the book who makes it into the film.