Think of it like the movie the last warrior, where the Vikings spoke Norwegian(ish) and gradually turned into English as he learned the language, to illustrate they still spoke Norwegian(they should have spoken Norse anyway, which is close to Icelandic) but he understood and spoke it now. After all he should have spoken Arabic not English himself so they do speak French, you just happen to speak French to
The 13th Warrior? Oh no why did you mention that, it was one of the most cringe-worthy scenes in movie history "i can speak your language now!" out of nowhere, Vikings all look at Banderas like he's nuts. That should be a meme, somehow.
Though that is basically anime logic...but why British English? Guess the operator is from the UK? That's something Ubi didn't address. Is this still even in the animus? Then who is the operator? Hope it's someone like the person from Black Flag, silent protagonist is a cliche, but at least it meant more room for Ed Kenway, who was awesome.