This is actually a bit difficult to provide definitive answer (the opinion of experts varies).
It might be worth checking some examples of the previous answers by me and others in these previous question threads:
A related question: in the game, the Saxons seem to unsurprised and often unalarmed by the sight of Vikings approaching close-by. Was the sight of vikings at this time common enough for the Saxons to be this casual about their approach?
As /u/y_sengaku notes, opinions vary, which is one reason I hesitated to respond. But y_sengaku's response to /u/EmptyMargins also raises a few specifics that I'd like to double down on.
First, excavators at the monastery of Portmahomack on the eastern coast of Scotland discovered two men (burials 129 and 153) who had been raised in Scandinavia (inferred from strontium and oxygen isotope analysis of their bones) and buried in the 700s (based on radiocarbon dating). Although it's not entirely clear what language would have been the lingua franca of the monastery, it's surprisingly definitive evidence that some Scandinavians were crossing the North Sea before the Viking Age.
Second, while we should be cautious about reading the Anglo-Saxon Chronicle too literally, its first report of viking violence at Portland in 787 hinges around the assumption that Scandinavians were already known as traders before the Viking Age. This fits the evidence of Portmahomack above, and it also fits what we know of Viking Age ship technology. In the later Viking Age it would be easy to tell the longship of a raiding party from a cargo-carrying knorr, but early Viking-Age ships like the Gokstad ship or Oseberg ship were both long enough to have a large crew and wide enough to carry a bit of cargo. That makes the story of the ill-fated reeve of Portland plausible. And it further suggests that there were ongoing interactions in the 700s where Scandinavians and Anglo-Saxons needed to find ways to be mutually intelligible.
And finally, as someone who researches in both Old English and Old Norse, I suspect an 18-year-old taken from Scandinavia and dropped in an Anglo-Saxon monastery would have understood very little indeed—at least at first. I studied Old Norse first and found the transition to Old English fairly easy, but I already had a broad background in medieval and modern languages. Certainly there were many people in the early Middle Ages who were even more comfortable with languages than I am. Frisian traders, for example, are known to have operated in both the North Sea and the Baltic, and the Norwegian chieftain Ohthere who visited King Alfred around 890 could probably communicate in Old Norse, Old English, and (I'd wager) Old Irish, if not Frisian as well.
If you think the early raiders were just country boys visiting from the fjords, then they'd lack the linguistic background necessary to make the jump from Norse to Old English. But if you think (as I do) that there were already well-traveled men among even the earliest groups of viking raiders, then there'd likely be individuals who could communicate without much apparent difficulty. By the time you get to the bigger raiding armies of the mid-800s (maybe by the 840s?), those were almost certainly cobbled together of groups from different regions, so many of those boys would have needed to develop agile ears as well. And by the 900s, most viking groups were basically colonizers who would have had long-term exposure to local languages. By 873, I think it's very plausible that a member of of viking band would not only be adept at languages in general but would likely have had some decent exposure to Old English as well.
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If you're interested in learning these languages on your own, I highly recommend checking out Viking Language and/or A Gentle Introduction to Old English.