The only languages I know, apart from bad comprehensive school "French", are so-called dead languages. I'm just waiting for an Anglo Saxon to come along who speaks perfect Anglian or a Viking. My degree would finally come in useful. (Well, it might have if I hadn't forgotten most of it). Am currently learning Latin (because am a sucker for punishment) but medieval ecclesiastical Latin as spoken by a monk might be a stretch too far. Would love to get an urgent phone call at 3AM from someone saying "My spirit guide's an Anglo Saxon warrior and has just turned up and is shouting "Hwaet!" a lot. Can you help?" but sadly there are no jobs in that.
Am guessing the way round the linguistic stuff for "mediums" is - they claim to see it in their heads as images. But am not sure how much of that a person from say, 1000 years ago, would manage to make mutually intelligible, given the difference in cultures, anyway? I mean, if he wanted to tell you to take a journey, he'd have to show you a picture of a horse, not your car, etc?