So I'm watching season 3 and something I don't understand is why suddenly the Vikings don't understand the Saxons and vice versa. In season 2 they communicated fine, everyone spoke English and they talked to each other with no issue. Now they need Athelstan to translate every word between them because they're each speaking their respective languages even though they could just each speak English when they had no trouble understanding each other before! I don't get it, it's driving me nuts. Like even when someone speaks English to the other why don't they understand them? Depends on which characters you're referring to.
For starters, the Vikings were not speaking English at the start of the series. They are speaking Old Norse. Their words are in English because it's a tv show, but by S3 it's been shown several times that the Vikings and Anglo Saxons are actually speaking their respective languages. They are by no means speaking Modern English.
