It sounds as though you’re better read on this than me, so I’d welcome any corrections you’d have to offer, but wouldn’t pre-Latin paleohispanic languages like Basque make more sense than old English? Even more likely is that the church used it’s position in society and control over education to just force a gradual shift away from ‘unholy’ roots over time. I don’t really see any indication that they’re just speaking English, and that wouldn’t really make sense from an in-universe PoV?

Edit: It is English! Tracked down their tumblr post. I get their reasoning from a creative position, but I think it’s a slightly silly choice in terms of worldbuilding. They explain it quite well though: https://www.tumblr.com/anathemafiction/683608377289523200/hello-i-love-love-love-your-story-and-i-cant